Walk into any conversation about modern investing and the letters "ETF" come up within minutes. Exchange-traded funds have become one of the most popular ways for UK investors to build a portfolio, whether inside a Stocks and Shares ISA, a SIPP, or a general investment account. Yet for someone opening a platform account for the first time, the mechanics of how an ETF actually works — and how it differs from a traditional fund — are not always obvious. This guide sets out what an ETF is, how it trades on the London Stock Exchange, and why so many investors now treat them as a default building block.
What exactly is an ETF?
An exchange-traded fund is a pooled investment vehicle that holds a basket of assets — shares, bonds, commodities, or a mixture — and is designed, in most cases, to track the performance of a specific index. A well-known example is a global tracker fund following the MSCI World Index, which holds shares in thousands of companies across developed markets in a single product. Rather than researching and buying each underlying holding individually, an investor can buy one ETF and gain exposure to the whole basket in a single trade.
The "fund" part
Like a traditional unit trust or OEIC, an ETF pools money from many investors and uses it to buy a diversified set of underlying assets. Ownership is divided into shares (or "units"), and the value of each share rises and falls with the value of the underlying basket.
The "exchange-traded" part
This is where ETFs differ fundamentally from traditional open-ended funds. An ETF is listed on a stock exchange — in the UK, typically the London Stock Exchange — and its shares can be bought and sold throughout the trading day, just like the shares of any listed company. A traditional fund, by contrast, is usually only priced and dealt once a day.
How ETFs trade on the London Stock Exchange
When an investor places an order for an ETF through a UK platform, that order is routed to the exchange and matched against other buyers and sellers, or against a market maker, in much the same way as buying shares in a listed company. The price an investor sees quoted throughout the day reflects live supply and demand, though it should stay close to the underlying value of the fund's holdings thanks to a mechanism involving "authorised participants" who can create or redeem large blocks of ETF shares to keep the market price aligned with the net asset value (NAV).
Real-time pricing versus once-a-day dealing
Because ETFs trade continuously, their price moves throughout market hours, reacting to news and market sentiment in real time. Traditional open-ended funds are priced once per day, usually based on the closing values of their holdings, meaning an order placed in the morning and one placed in the afternoon will typically be filled at the same single price.
Bid-offer spreads
Because ETFs trade like shares, there is a bid price (what buyers are willing to pay) and an offer price (what sellers want), with a small gap — the spread — between them. Highly liquid, popular ETFs tend to have narrow spreads, while niche or thinly traded ETFs can have wider ones, which is an additional, sometimes overlooked, cost of dealing.
How ETFs differ from traditional funds
Both ETFs and traditional funds (OEICs and unit trusts) can track the same index, and in many cases hold near-identical underlying assets. The practical differences for a UK investor tend to come down to how they are bought, priced, and sometimes charged for.
| Feature | ETF | Traditional fund (OEIC/unit trust) |
|---|---|---|
| Trading | Continuously during market hours, on an exchange | Once per day, at a single valuation point |
| Pricing | Live market price, close to NAV | Single daily NAV price |
| Dealing costs | Bid-offer spread, plus platform trading commission | Usually no explicit dealing charge on many platforms |
| Minimum investment | Cost of one share (or fraction, if supported) | Often allows regular saving of small fixed amounts |
| Typical ongoing charge | Often very low for mainstream index trackers | Can be low for index trackers, higher for actively managed funds |
Neither structure is inherently "better" — the right choice often depends on how an investor wants to trade, whether they are investing a lump sum or a regular monthly amount, and which platform and account they use.
Why ETFs have grown so popular with UK investors
Low ongoing charges
Many of the most widely held ETFs are simple index trackers, and because tracking an index requires relatively little active decision-making, their ongoing charges figure (OCF) tends to be low compared with actively managed funds. Over long holding periods, small differences in annual charges can compound into a meaningful difference in final portfolio value.
Broad diversification in one trade
A single ETF can provide exposure to an entire market, sector, or region, which can help investors build a diversified portfolio without needing to buy dozens of individual shares or funds.
Transparency
Most ETFs publish their full list of holdings daily, giving investors a clear view of exactly what they own, in contrast to some actively managed funds that disclose holdings less frequently.
Flexibility of trading
Being able to buy or sell at any point during market hours suits investors who want more control over the exact price at which they deal, although for long-term, buy-and-hold investors this intraday flexibility may matter less in practice.
Things to understand before investing in an ETF
Not all ETFs are simple trackers
While many ETFs passively track a broad index, others are more specialised — following a narrow sector, a theme, or using more complex strategies. It is worth reading a fund's factsheet and Key Investor Information Document (KIID or KID) to understand exactly what it holds and how it behaves.
Physical versus synthetic replication
Some ETFs hold the underlying assets directly ("physical" replication), while others use derivatives to replicate an index's returns ("synthetic" replication). This distinction carries different risk considerations, covered in more depth elsewhere on this site.
Currency exposure
An ETF tracking an overseas index, such as the S&P 500, will typically expose a UK investor to movements in the exchange rate between sterling and the foreign currency, unless the investor specifically chooses a currency-hedged share class.
Accumulating versus distributing share classes
Many ETFs offer a choice between an "accumulating" version, which reinvests income automatically, and a "distributing" version, which pays income out to the investor, typically as cash. The right choice often depends on the account type and whether an investor wants automatic reinvestment or a regular income stream.
A worked example: comparing costs over time
Consider a hypothetical investor who invests £10,000 in a global equity ETF with an ongoing charge of 0.15% a year, compared with investing the same amount in an actively managed global fund charging 0.90% a year. Assuming, purely for illustration, that both investments grow at an identical 6% a year before charges over 20 years, the ETF investor would pay roughly £460 in charges in year one, rising as the pot grows, while the actively managed fund investor would pay around £2,700 in year one on the same basis. Compounded over two decades, this difference in charges alone — even before considering any difference in investment performance — could amount to several thousand pounds. This is a simplified, hypothetical illustration only, not a forecast, and actual returns and charges will vary.
Buying an ETF within a tax-efficient account
Most UK platforms allow ETFs to be held within a Stocks and Shares ISA (annual allowance £20,000 for the 2025/26 tax year, across all adult ISA types combined) or a Self-Invested Personal Pension (SIPP), sheltering any gains and income from Capital Gains Tax and, in the case of a SIPP, providing tax relief on contributions. Held outside a tax wrapper, gains above the £3,000 annual Capital Gains Tax exempt amount may be taxable, and dividend income above the £500 annual dividend allowance may also be taxable. These figures apply for the 2025/26 tax year, and readers should always check the current HMRC and FCA figures, as allowances and rates are reviewed and can change.
How an ETF's price stays close to its underlying value
New investors sometimes wonder what stops an ETF's market price from drifting far away from the actual value of the assets it holds, given that it trades on an open market like any listed share. The answer lies in a mechanism involving specialist market participants known as "authorised participants" (APs), usually large banks or trading firms.
The creation and redemption process
If an ETF's market price rises noticeably above the value of its underlying holdings, an authorised participant can buy the underlying basket of assets, deliver it to the ETF provider in exchange for newly created ETF shares, and then sell those shares in the market at the higher price — a profitable trade that increases the supply of ETF shares and pushes the price back down towards fair value. The reverse happens if the ETF's price falls below its underlying value: an AP buys the cheaper ETF shares, redeems them with the provider in exchange for the underlying basket, and sells the underlying assets, reducing the supply of ETF shares and nudging the price back up. This constant arbitrage opportunity is what keeps ETF prices closely aligned with net asset value in normal market conditions, even though the fund itself does not set the price directly.
When this mechanism can be tested
In periods of extreme market stress or very low liquidity in the underlying assets, this alignment mechanism can occasionally become less efficient, and a wider-than-usual gap between an ETF's market price and its underlying value can briefly appear. This is more commonly observed in ETFs holding less liquid underlying assets, such as certain corporate bonds, than in ETFs tracking highly liquid large-company equity indices.
Choosing between similar ETFs from different providers
Once an investor has decided on a particular market or index to gain exposure to, there are often several competing ETFs available tracking the same or a very similar benchmark. A few practical points of comparison can help narrow the choice.
Fund size and liquidity
Larger, more established ETFs tend to have narrower bid-offer spreads and higher trading volumes, which can reduce the practical cost of dealing, particularly for larger trade sizes.
Ongoing charges figure
Even among ETFs tracking the same index, the ongoing charges figure can vary between providers, and this difference compounds over time, as illustrated in the worked example below.
Tracking difference
Beyond the headline ongoing charge, it is worth checking how closely a fund has actually tracked its benchmark historically (its "tracking difference"), since replication method, sampling decisions, and securities lending income can all cause a fund's real-world return to differ slightly from both its stated charge and the index itself.
Domicile and reporting fund status
Many ETFs available to UK investors are domiciled in Ireland or Luxembourg for tax efficiency reasons, and it is worth confirming a fund holds UK HMRC "reporting fund" status, which affects how any gains are taxed when the fund is eventually sold outside a tax-efficient wrapper.
Key takeaways
- An ETF is a pooled fund that trades on a stock exchange throughout the day, unlike traditional funds which are priced once daily.
- ETFs often carry low ongoing charges, particularly when tracking broad market indices such as the FTSE All-Share or MSCI World.
- Buying an ETF typically involves a bid-offer spread and a platform dealing charge, which is a different cost structure from many traditional funds.
- Not all ETFs are simple index trackers — some use synthetic replication, follow niche themes, or carry currency exposure worth understanding before investing.
- Choosing between accumulating and distributing share classes, and using tax-efficient accounts such as ISAs and SIPPs, are both worth considering as part of the wider investment decision.
- Always check current HMRC and FCA figures, as tax allowances and thresholds are reviewed regularly and may change.