New ISA Money in 2026: Your FTSE 100 Loyalty Costs You 3.3% a Year. Buy the S&P 500.

The S&P 500's long-run annualised return of roughly 10.5% against the FTSE 100's 7.2% is not one decade's fluke; it is the compounding of an economy that owns its technology companies. The gap looks small on a page and enormous on a chart: £10,000 becomes about £200,000 in the S&P and about £80,000 in the FTSE over thirty years.

A valuation discount that persists for four decades is not a discount — it is a price. The FTSE has traded cheaper than America for most of that period and still returned less. Cheap can stay cheap for longer than you can stay solvent, which is the uncomfortable part the income argument skips.

Add monthly contributions and the gap yawns wider. £500 a month into the S&P 500 at 10.5% grows to about £1.1 million over thirty years; the same £500 into the FTSE 100 at 7.2% reaches roughly £600,000. The difference is not the first £500 or the last. It is the three-point annual gap doing its quiet work every single year for three decades.

The last twelve months look close: the FTSE returned 13.9% on price and the S&P 14.5% in dollars. One year is noise. Four decades is the signal. Zoom out and the gap returns year after year, because the American earnings machine simply grows faster.

The FTSE 100's heaviest sectors are banks, oil, mining and tobacco. Its market capitalisation of £2.49 trillion is anchored by AstraZeneca and HSBC, with no meaningful home-grown technology platform to speak of. The companies that did scale — Ashtead, CRH and Flutter — delisted from London to New York in 2024 in search of higher valuations, taking around £120 billion of market value with them.

That is the structural story in one sentence: Britain produces profitable but low-growth companies, and the world's growth companies list in America. Buying the FTSE 100 is not a bet on the UK economy recovering; it is a bet on banks and commodities continuing to grind out income.

And the FTSE 100 is not even a clean bet on Britain. Its companies are mostly international, which is why the FTSE 250 is the better gauge of the domestic economy. Backing Britain with a FTSE 100 tracker really means backing global banks, oil prices and a London listing — not the UK.

Look at what the index is missing. There is no Nvidia, no Apple, no Microsoft equivalent in London; the closest is Sage, and its weight is a rounding error next to America's giants. A UK investor who stayed home missed the single largest source of global equity returns for a decade.

If you want the global growth engine, you have to buy the index that holds it. The S&P 500 is that index, and it is available inside any stocks and shares ISA.

A high dividend yield is often the market telling you it expects slower growth — or outright decline. The FTSE's 3.1% yield is partly income and partly a warning. The S&P's 1.06% yield is low because its companies are reinvesting earnings into growth that has, so far, delivered.

Think of it as the market's honest pricing. When a company yields 7% or more, the market is not giving you free money — it is pricing in a sector in decline. Dividend yield is a valuation signal, not a coupon, and reading it as a coupon is how income investors get trapped.

There is a reason income investors cluster in the FTSE and growth investors cluster in the S&P: the two indices sell different things. One sells cash today; the other sells earnings tomorrow. Over a twenty-year horizon, cash today is the smaller prize — you can always convert growth into income later by selling a sliver of a bigger pot.

I will take the capital growth and buy income later if I need it. You can sell shares to pay yourself; you cannot grow a dividend stream into a technology company.

None of this says income is worthless. It says income is not a substitute for growth. The FTSE 100 pays you 3.1% to sit still; the S&P 500 makes you work through the volatility but has historically handed you the larger pot at the end. For a saver with two decades still to run, the second deal has been the better one almost every time.

Sterling strengthened 2.8% against the dollar over the last twelve months, which clipped US returns for UK investors. Currency is a real cost — but it is also exactly why you diversify. The pound's long-run trend against the dollar has been down, and the dollar remains the world's reserve currency and the denomination of most global trade.

A UK investor holding only the FTSE 100 has no currency hedge at all: every pound of their ISA is exposed to one currency, one tax regime and one economy. The S&P 500, held inside an ISA alongside the home market, gives you a second economy and a second currency without leaving your tax wrapper.

The Bank of England has cut the base rate from 4.25% in May 2025 to 3.75%. A falling UK rate is a slow headwind for the pound and another reason not to keep every pound of capital in one currency. When your own central bank is easing while your savings are entirely sterling, the diversification case for holding some dollar assets writes itself.

The Bank of England base rate of 3.75% sets the local alternative. If you are weighing US equities against UK cash, remember the comparison that matters is a decade out, not a quarter.

The S&P 500 trades at 26.16 times trailing earnings. That is expensive by history, and I will not pretend otherwise. But the multiple reflects something real: the index now holds the world's most profitable technology franchises, and those earnings are still growing.

The alternative is not ‘buy the FTSE instead and dodge the valuation problem.’ It is ‘pay 16 times for a lower-growth, single-country, commodity-heavy index and call it prudence.’ Prudence at the wrong price is just another name for underperformance.

History says paying a high multiple hurts your next-decade returns, but it does not tell you when. The investors who avoided America for the last ten years because it was expensive are the same ones who missed the best decade of returns on offer. Valuation is a compass, not a stopwatch — and a compass that has pointed ‘avoid’ for a decade has been pointing the wrong way.

The S&P 500 is not really an American bet anymore. Roughly 40% of its revenues come from outside the United States, so buying the index buys a slice of global growth with American governance. The FTSE 100 is similar on paper — most of its revenues are overseas too — but those revenues come from commodities, banks and consumer staples rather than software, chips and payments. One index sells the world raw materials and loans; the other sells the world the tools it runs on. The second business has compounded far faster, and the multiple gap is the market pricing in which business it expects to keep winning.

The FCA's diversification guidance exists because concentration is the risk — and a UK-only ISA is the concentrated bet, not an S&P 500 tracker. Understand volatility before you buy, then size the position to your tolerance.

You get £20,000 of ISA allowance in 2026/27. I would put the bulk of it to work in a global or US tracker that owns the S&P 500 — not as a trade, but as the permanent core. A simple index fund or ETF does the job for a few basis points.

Keep some FTSE 100 exposure if the income matters to you, but treat it as the satellite, not the planet. The evidence on active versus passive says most fund managers fail to beat their benchmark — so do not pay someone to underperform the American index for you.

Set it up once and stop checking. The monthly direct debit into a global tracker, the dividend reinvestment switched on, the ISA wrapper doing its tax-free work — that is the entire machine. The debate between the FTSE and the S&P matters less than whether you actually stay invested for the thirty years that make the maths work. The investing hub has the deeper map.