Two men buy the same index on the same day. Twelve months later, one account shows a return of roughly 18%. The other shows roughly 8%. Neither made a bad call, checked the wrong price, or panic-sold at the worst moment — they simply held different share classes of the same S&P 500 tracker, and the pound did the rest.
The index doesn't care what currency you think in
The S&P 500 is priced in US dollars. Every constituent company reports earnings in dollars, every price tick on the index moves in dollars, and the total return figure you see quoted in the financial press is a dollar number. When you buy a UCITS ETF tracking that index through a UK platform, your sterling gets converted into dollars at the point of purchase, the fund does its job of tracking the index in dollars, and then — if you're holding an unhedged share class — the value gets translated back into sterling every time you check your balance. That translation step is where currency risk lives, and it has absolutely nothing to do with whether Apple, Microsoft or Nvidia had a good year.
Sterling has spent most of 2026 trading against the dollar in a range between roughly $1.33 and $1.37, with the pair sitting around $1.34–$1.36 through mid-to-late August. Go back to 2022 and GBP/USD dropped close to parity with the dollar during the autumn mini-Budget crisis before recovering. That's a swing of well over 25% in the exchange rate alone, layered on top of whatever the S&P 500 itself did — and a UK investor holding an unhedged tracker through that period lived through both moves at once, whether they meant to or not.
A tracker fund can nail its benchmark to the decimal and still hand a UK holder a return that has nothing to do with what the S&P 500 actually did that year.
This isn't specific to one index. A Nasdaq-100 tracker carries the same dollar exposure and the same conversion step. So does a global tracker like an MSCI World or FTSE All-World fund, because roughly 65–70% of those indices by weight is US-listed and therefore dollar-denominated, even though the fund itself might be priced in sterling on your platform screen. You can hold a fund with "World" in the name and still be carrying a dollar-heavy currency bet without ever having chosen one. A few signs you're already carrying this exposure without realising it: your platform quotes the fund's factsheet return in USD or shows a "currency effect" line on the annual statement; the fund's top ten holdings read like a list of US mega-caps; the fund provider offers a separate "GBP Hedged" or "Hedged Share Class" version of the same product on the same platform — that second listing existing at all is the tell.
Same index, two tickers, two returns
You can see this play out directly by comparing two real, UK-listed products tracking the identical benchmark. The iShares Core S&P 500 UCITS ETF (ticker CSP1) is unhedged — it charges a 0.07% ongoing charge and simply lets the dollar exposure ride. The iShares S&P 500 GBP Hedged UCITS ETF (ticker IGUS) tracks the same underlying index but uses currency forwards to strip out the GBP/USD swing, and it charges a 0.20% ongoing charge for the privilege. In periods when the dollar has strengthened against the pound, IGUS has tracked the S&P 500's dollar return almost exactly while CSP1 has lagged behind it by roughly the size of the currency move; in periods when the pound has strengthened instead, the relationship flips and the hedged share class is the one giving something up. Vanguard, for what it's worth, doesn't even offer a GBP-hedged share class for its popular S&P 500 UCITS ETF (VUAG) — for equity index funds specifically, most UK platforms simply don't stock the hedged option, which tells you something about how niche the demand for it actually is.
The mechanics of the hedge matter as much as the headline fee. IGUS and funds like it don't just buy a currency-neutral basket — they roll forward contracts every month to lock in an exchange rate, and the cost of doing that isn't fixed. It moves with the gap between UK and US interest rates, which is exactly the kind of variable most retail investors never think to check before buying a "safer" version of a fund they already understand.
What the hedge actually costs you
Add the OCF gap to the rolling cost of the forward contracts and a GBP-hedged S&P 500 share class typically costs somewhere in the region of 0.10–0.30 percentage points a year more than its unhedged twin, depending on prevailing rate differentials. That doesn't sound like much until you hold it inside a Stocks and Shares ISA for twenty years, where even small, compounding annual drags meaningfully change the terminal value of a portfolio funded with £20,000 a year — the full 2026/27 ISA allowance. Hedging doesn't change the expected long-run return of the underlying index; it only changes the volatility you experience getting there, and you're paying every single year for a smoother ride whose value only shows up in the years currency moves against you.
Here's the part that gets missed: sterling's long-run drift against the dollar has actually worked in unhedged holders' favour for most of the past decade, even accounting for the sharp rebound off the 2022 lows. A UK investor who bought an unhedged US tracker in, say, 2015 has collected a currency tailwind on top of the index return more often than not. That's not a law of nature — it's a historical pattern that could reverse — but it's the reason most UK platforms default new ISA and SIPP buyers into the unhedged share class rather than steering them towards the hedged one.
The tax trap that has nothing to do with currency
Currency exposure isn't the only thing that differs between two funds tracking the same index — and this next part catches out UK investors who think they've done their homework. HMRC maintains a list of offshore funds, including the vast majority of Irish- and Luxembourg-domiciled UCITS ETFs, that hold Reporting Fund Status. A fund on that list reports its income to HMRC and to investors every year, which means gains on disposal are taxed under normal Capital Gains Tax rules — 18% for basic-rate taxpayers, 24% for higher and additional-rate taxpayers on most assets, after the £3,000 annual exempt amount. A fund that has never held Reporting Fund Status, or that lost it at any point during your holding period, triggers something different entirely on sale: an "offshore income gain," taxed as income at your marginal rate — up to 45% for additional-rate taxpayers — with no CGT allowance to offset it at all.
In practice, this rarely bites UK buyers of the big, obvious S&P 500 UCITS ETFs, because CSP1, IGUS, VUAG and their peers are all domiciled in Ireland and all hold Reporting Fund Status — checking the list takes two minutes and it's worth doing before you commit £20,000 of ISA allowance to anything, not after. Where it does bite is with smaller, more obscure offshore-domiciled funds that a platform lists but that never bothered to apply for the certification, or funds that let their status lapse without anyone noticing. Check the HMRC reporting fund list before you buy a fund you haven't heard of before — not after you've already held it for three years and are staring at a disposal that HMRC wants to tax as income.
When hedging earns its keep
None of this means hedged share classes are a bad product — they're the right tool for a narrower job than most people use them for. If you're holding US equity exposure inside an ISA specifically because you'll need that money in three to five years — a house deposit, a wedding, school fees due on a fixed date — the hedged share class removes one entire source of uncertainty from an amount you can't afford to have swing against you right before you need it. That's a legitimate, deliberate trade of extra ongoing cost for reduced short-term variance, and it's the one scenario where paying the 0.10–0.30 percentage point premium makes sense.
For anything sitting inside a Stocks and Shares ISA or SIPP with a decade or more left to run before you touch it, skip the hedged share class. The extra fee compounds every year regardless of which way the pound happens to move, and over a long enough horizon the currency swings that the hedge is protecting you from tend to average out anyway. Buy the unhedged tracker, hold it through whatever GBP/USD does next, and spend the time you would have spent worrying about the exchange rate checking that the fund is still on HMRC's reporting list instead — that's the box actually worth ticking twice.
The same logic extends to a Junior ISA, where the time horizon is by definition long — a JISA opened at birth has eighteen years to run before the child can touch it, well past the point where any single year's currency swing matters against the compounding. With the JISA allowance sitting at £9,000 for 2026/27, a parent funding US equity exposure inside one is about as far from a three-to-five-year cash need as it's possible to get, and paying an extra 0.10–0.30 percentage points a year for currency smoothing there is money spent solving a problem that doesn't exist yet.