Ask two UK investors what "the UK market" means and you may get two different answers. One will point at the FTSE 100, the blue-chip benchmark quoted at the end of every news bulletin. The other will point at the FTSE 250, where the mid-caps live. They share a trading venue and a country, but they behave like distant cousins: different industries, different currencies, different moods. Knowing which is which changes how you read headlines and where you put your money.
The FTSE 100 tracks the 100 largest companies by market capitalisation on the London Stock Exchange's Main Market. Market capitalisation is share price multiplied by the number of shares in issue, so the biggest businesses carry the most weight — a handful of giants can move the whole index.
The FTSE 250 takes the next 250 companies below that line. Put the two together and you get the FTSE 350; add the small-cap universe and you have the FTSE All-Share, which covers the overwhelming majority of the Main Market by value.
A place in the 250 is not a runner-up prize; these are still substantial businesses, often worth billions. Membership is reviewed quarterly, and companies are promoted or relegated as their valuations shift. That matters more than it sounds, because tracker funds must buy the promoted names and sell the demoted ones, often around the same dates.
Look past the numbers and the split becomes obvious. The FTSE 100 leans on energy, mining, banking, insurance, pharmaceuticals and consumer staples — businesses with global reach. The FTSE 250 looks more like the domestic economy: housebuilders, industrials, retailers, pub and leisure groups, transport operators, specialist lenders, healthcare and technology names, alongside a fair number of listed investment trusts.
That explains why the two react to different news. A commodity price move or a shift in Chinese construction demand tends to register in the 100. A Budget, a change to stamp duty, a UK consumer confidence reading or a Bank of England rate decision tends to register more sharply in the 250.
Because the FTSE 100 is weighted by size, its performance can be driven by a dozen or so companies. Own a FTSE 100 tracker and you may be taking more single-stock risk than you realise. The 250 spreads money across far more names, so no individual company dominates — though each holding is smaller and trades less heavily.
Most FTSE 100 constituents earn the bulk of their revenue overseas, in dollars, euros and other currencies, even though their shares are priced in sterling. When sterling weakens, those overseas profits translate into more pounds, which can flatter reported earnings. When sterling strengthens, the effect reverses.
The FTSE 250 leans the other way. Mid-caps generally generate more of their revenue in the UK, so translation effects are smaller and domestic conditions matter more. It is not a clean split — plenty of mid-caps export, and plenty of blue-chips have UK operations — but the broad direction holds.
So a FTSE 100 tracker is not a straightforward bet on the British economy. It is a global earnings stream priced in sterling. If you want exposure to the UK as a domestic story, the 250 or an All-Share fund is often a better fit.
Mid-caps have historically been the more volatile of the two. They are smaller, often more cyclical, more sensitive to domestic credit conditions and traded less heavily, all of which amplifies moves in both directions. In risk-off periods the 250 has tended to fall harder; in recoveries it has often bounced further.
The FTSE 100 carries a defensive skew, with healthcare, consumer staples and energy companies that tend to hold up better when growth worries dominate. It also tends to offer a higher dividend yield, one reason it stays popular with income-focused investors.
None of this is a law of nature. A banking crisis or a collapse in commodity prices hits the 100 hard, and a domestic recovery can leave global earners behind. Treat the pattern as a tendency, not a guarantee.
Start with what you actually want. If you are after global earnings and income, a FTSE 100 fund may do the job. If you want exposure to the UK's domestic economy, look at the 250 or the All-Share. Plenty of investors hold both and accept the overlap with the wider market.
Then check the details. Fund factsheets show which index a fund tracks, its top ten holdings and how many companies it owns; two funds with similar names can look very different inside. Costs and tracking difference matter too, particularly over long holding periods.
Think about currency at portfolio level rather than fund level. If most of your other investments are already priced in dollars, adding a globally earning FTSE 100 tracker does not add much diversification on that front. And remember that index reshuffles create forced buying and selling — long-term investors can ignore most of it, but it explains why a holding may appear or vanish from your fund without any decision on your part.
Both indices have a place in a sensible portfolio, and neither is automatically better. The useful question is not which one wins, but which one does the job you need it to do. This is general information rather than personal financial advice, so if you are unsure how it fits your circumstances, speak to a regulated financial adviser.
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