.CO.UK is cheaper than a hamburger. I would not go for 'cheapest' because that cheapest registrar will be less than 1£ cheaper than an established registrar. Go for quality with an established registrar, you won't pay more than a few pounds a year anyway.
Be aware though, apparently paperworks (a paper to sign and send back by post) is needed to transfer ownership to someone else. Which I like a lot because I mainly register for personal use and hate the idea of theft, but for sellers it may make .co.uk less attractive.