The iShares FTSE 100 ETF. This is quoted on the London Stock Exchnage can be bought and sold through any stockbroker. It's a fund which actually holds shares in the companies forming the index in proportion to their index weighting.
You can spreadbet on the nominal index value. You don't own shares, you're just betting on the rise or fall of the index. This is a tax effiicient method as gains are tax-free. It also allows you to short the index which you cannot do with the ETF.