FTSE (Financial Times Stock Exchange) is a family of indices run by FTSE Russell that track different groups of shares by market, sector, size, or theme. It is not a single index.
The best-known FTSE index is the FTSE 100, the 100 largest companies on the London Stock Exchange by market capitalisation. The family also includes the FTSE 250, the next 250 companies by size, the FTSE 350, the two combined, and the FTSE All-Share.
Traders and investors use FTSE indices to gauge UK market performance at different size tiers and to trade broad UK exposure through index funds, exchange-traded funds, futures, options, or CFDs.
You want UK exposure across company sizes.
The FTSE 100 gives you the largest blue chips, the FTSE 250 gives you mid-caps, and the FTSE All-Share covers nearly the whole London market. Choosing among them sets how much large-cap versus mid-cap risk you take.