www.onandone.co.uk you'll need to host your site with them as well although they are inexpensive for that too.
http://www.123-reg.co.uk/ for .co.uk, £3.04 a year, minmum 2 years. godaddy for .com I think it cost me about £21 for 5 years. 123 are very fast and have a friendlier control panel than godaddy. It's very easy to get ripped off for domain names and hosting. Cheapest isn't always best though, esp for hosting, customer service is very variable. I use these guys for hosting: http://www.qualityhostonline.com/index.htm I grabbed 200Mb (should only have been 100MB, you can't get the 200Mb now) and 2GB/month for $6.60 a year! If you want more space use this comparison site as those guys aren't cheapest: http://www.findmyhosting.com/