* do they look likely to stay in business? * how good is the support? * who provides DNS, you or them? * hidden costs, e.g. they charge you if you want to transfer the domain to another registrar, they charge you if you want to manage your own DNS, they charge you if you want web forwarding... * you might want to register the domain with the same firm that runs your web hosting and email, it can make life simpler if you only have to deal with one company.
Uk domains are registered for 2 years and cost £5+VAT (£5.88) if you are a member of Nominet (the official UK domain body) so any company that offers domains for less than this going to make a loss unless they have other ways of clawing it back somehow.
One of the biggest companies that people use in the UK is 123-reg, they charge £0.18+VAT on top of the Nominet fee and have NO hiiden costs that I have found.
If you are going to register a lot of UK domains then you could become a member of Nominet yourself but it will cost you about £600+VAT