Your FTSE 100 ISA Has Made 7.2% a Year — the S&P 500 Made 10.5%. That Gap Cost You £127,000

Since 1986, the FTSE 100 has delivered roughly 7.2% annualised total return in sterling terms. The S&P 500, over the same 40-year window: about 10.5% annualised. That 3.3 percentage point gap sounds modest. Over a working life, it's devastating.

Take a 30-year-old ISA investor putting away the full £20,000 allowance each year. After 30 years, the UK-only portfolio would be worth roughly £1.89 million. The global portfolio: £2.76 million. That's an £870,000 shortfall from staying home — enough to buy a house outright in most of Britain.

Even for a one-off £50,000 lump sum left to compound for 25 years, the difference is £127,000. The gap isn't about one bad year. It's about the slow, invisible erosion of returns that nobody feels month to month but everyone pays over a lifetime.

The performance gap isn't new, and it isn't closing. The FTSE 100 was at 6,930 in December 1999. On 10 June 2026, it sits roughly 20% higher. The S&P 500 has roughly quadrupled in the same period. That's not a cycle. It's a structural divergence.

Why? The FTSE 100 is heavy on banks, oil majors, miners, and tobacco — sectors the market has systematically re-rated lower for two decades. The US index is dominated by technology and healthcare — sectors that compound earnings at double-digit rates. You can't fix sector composition with patience. For a deeper look at what drives FTSE 100 income, see our dividend yield explainer.

The MSCI All-Country World Index weights the UK at roughly 3.8%. The average UK retail investor holds somewhere between 35% and 50% of their equity portfolio in UK-listed stocks, depending on which survey you read. That's not a tilt. That's a structural bet against the rest of the world.

Why do investors do this? The FCA's own research points to familiarity bias: people invest in companies they've heard of. Most Britons can name five FTSE 100 companies. Most can't name five companies in the MSCI World ex-UK.

But here's the uncomfortable truth: those familiar names have been terrible investments. Barclays trades at £4.45 with a P/E of 10.3. HSBC at £12.79. These are not broken companies — but they've been value traps for over a decade. The market isn't wrong about them. It's pricing in structural headwinds: regulation, low growth, and the slow decline of legacy banking models.

Shell at £31.84 with a 3.6% yield looks better on paper, but ask yourself: in a world transitioning off hydrocarbons, do you want 15% of your ISA in two oil majors? Because that's what the FTSE 100 gives you — Shell and BP together are roughly 15% of the index by weight.

Meanwhile, you have almost zero exposure to the companies reshaping the global economy: the semiconductor firms, the AI platforms, the biotech innovators. Those companies happen to list in New York, not London. Avoiding them isn't cautious. It's a bet that the 20th century's industrial structure will outperform the 21st's. If you're new to building a global portfolio, our guide to index funds and ETFs walks through the practical steps for UK investors.

The standard defence of home bias is currency risk: "If I invest abroad and sterling strengthens, my returns get crushed." This sounds prudent. It's actually backwards.

Sterling has depreciated against the dollar by roughly 30% over the past 20 years. The pound bought $1.90 in 2006. It buys about $1.34 today. Every British investor who stayed home during that period lost purchasing power on everything priced in dollars — which is most of what you buy: energy, technology, food commodities.

Investing globally is a hedge against sterling weakness, not a gamble on it. If the UK economy performs well and sterling strengthens, great — your salary, your house, and your state pension are all sterling-denominated and benefit. If the UK underperforms and sterling weakens, your global investments cushion the blow.

The real currency risk is having all your assets in the same currency you earn and spend in. That's concentration risk dressed up as prudence. A UK investor with a global portfolio is actually more diversified across economic scenarios than one with a UK-only portfolio, precisely because of — not despite — currency exposure. The Office for Budget Responsibility publishes long-term sterling forecasts that make the structural case for currency diversification clear.

The one argument for UK equities that has genuine merit is income. The FTSE 100 yields roughly 3.5%, compared to about 1.3% for the S&P 500. For a retiree drawing income, that difference is meaningful.

But dividend yield is not free money. It's a return of capital that the company chose not to reinvest. High-dividend markets tend to be low-growth markets — the companies can't find better uses for the cash internally. That's exactly the FTSE 100 story: mature, slow-growing businesses returning capital because they've run out of organic growth opportunities.

More importantly, total return is what matters. If the S&P 500 returns 10% with a 1.3% yield, you can sell 2.2% of your holding each year and still have 8.8% in capital growth. If the FTSE 100 returns 7% with a 3.5% yield, you're left with 3.5% growth after taking the same 3.5% income. Over a 30-year retirement, that difference compounds catastrophically. Our EPS and dividend cover guide explains how to spot dividends that won't survive a downturn.

The HMRC dividend allowance of £500 is a nice feature, but it shelters less than a year's growth difference between the two markets. You're optimising for tax on £500 of income while leaving thousands in capital returns on the table. For higher-rate taxpayers, the situation is worse: dividend income above the allowance is taxed at 33.75%, which means a 3.5% UK yield delivers only 2.3% after tax for a higher-rate investor maxing out their allowance. Suddenly the FTSE 100's headline income advantage shrinks dramatically.

The Office for National Statistics CPI data shows inflation running at roughly 3.3% over the past year. A 3.5% dividend yield that barely beats inflation and gets taxed at 33.75% above £500 is wealth preservation, not wealth creation. The global portfolio's growth engine — however imperfect — gives you a shot at real returns.

None of this means you should hold zero UK equities. Global market-cap weight — roughly 4% — is the neutral starting point. Most UK investors can justify a modest home tilt of 10–15% for genuine reasons: dividend withholding tax on foreign shares inside an ISA can't be reclaimed, UK dividends have a £500 tax-free allowance, and some lean towards UK smaller companies where home advantage is real.

But 40–50%? That's not a tilt. That's a concentrated bet on a single country that constitutes one twenty-fifth of global market value. No financial adviser would recommend putting half your ISA into German stocks, or Japanese stocks, or any other single country. The UK should be no different.

The simplest fix: switch your core ISA holding from a FTSE 100 tracker to a global index fund. The Vanguard FTSE All-World ETF or the iShares MSCI World ETF give you exposure to 1,500–2,000 companies across developed markets. Add a FTSE 250 tracker at 10% if you want UK small-cap exposure where the home advantage is plausible. That's it. Two funds. Done. See our guide to building a diversified portfolio for a practical framework.

The active vs passive debate is settled for most investors: 9 out of 10 active UK fund managers trail their benchmark. A low-cost global tracker removes the active manager risk and the home bias risk in one trade. Browse our investing hub for more on building a globally-diversified portfolio from a UK ISA. Your future self — the one trying to retire in 2056 — will not look back and wish you'd owned more banks and oil companies.