For a walkthrough of the cheapest way to buy Bitcoin (and other cryptocurrencies) in the UK, check out the guide linked above.
Solidi is a UK-based peer-to-peer trading site which allows you to buy Bitcoin at competitive rates with a UK bank transfer.
The site allows users to post advertisements where they state exchange rate and payment methods for buying or selling bitcoins. Unlike centralised exchanges, you’ll be dealing directly with another person on LocalBitcoins.
Established in 2012, LocalBitcoins.com remains a popular option for buying and selling Bitcoin around the world.
Bitstamp allows you buy Bitcoin, Ethereum, Litecoin, and Ripple with US Dollars or Euros. Fees start at 0.25% but go as low as 0.1% depending on your volume.
Coinfloor is a UK-based Bitcoin exchange which allows deposits of GBP. The minimum deposit is £2500.
Revolut recently announced an in-app cryptocurrency exchange which is a fast and easy way to buy, hold, and exchange Bitcoin, Ethereum, and Litecoin at the best possible exchange rate. In its current state, Revolut does not allow you to withdraw your cryptocurrency from their application and cryptocurrency holdings are not insured.
This feature is currently being rolled out to all users. Find out more here.
Find a local ATM near you to buy cryptocurrency.