Localbitcoins would be the easiest method but most traders require identity verification when buying coins from them. They might not need you to do it if you will sell.
If you are not in a hurry, apply for Cryptopay visa card. The advantage is you can use their app to easily convert your coins to GBP which you then load into your card. It's a quick procedure and all done in under a minute. All UK banks honor Cryptopay's visa card. If you fully verify your Cryptopay account, you can withdraw up to £1,600 per day:
https://cryptopay.me/bitcoin-debit-cardAlternatively, there are bitcoin ATMs in London which buy coins. It will dispense cash but this is something I would not recommend. Their rates are horrible.