You can use gilt listings on Hargreaves Lansdown or Interactive Investor's sites to find a gilt with a combination of maturity date, price and coupon that suits you. Then use our calculator to see what the rate of return is on your chosen gilt.
If you can't choose, then some platforms offer what are known as 'gilt funds'. You choose to invest in these, and the fund manager will choose which gilts the fund invests in, with the aim of maximising the return on the fund. Do make sure if you choose this route that you know exactly what the fund invests in – some will also add corporate bonds, which increases the risk.
It's also worth noting that if you opt for a gilt fund (rather than investing in individual gilts), then any capital gain you make is subject to normal capital gains rules, so you may need to pay tax on the gain (unless the gain's within your annual £3,000 allowance, or the fund is held within an ISA).
If you really aren't sure you can seek help from an independent financial adviser who will advise you on whether gilts are a good option for you, and – if so – which specific gilts might suit your purposes.