.CO.UK is cheaper than a hamburger. I would not go for 'cheapest' because that cheapest registrar will be less than 1ยฃ 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.