UKreg looked quite promising: cheap prices, and a handy tool to check on which domain names are available and suggest options. In order to register a domain name, you need to have an account, so I went through the steps of creating one. The final step says:
We need your card details to activate your account.
We will not charge your card at this stage.
I put in the company credit card, but that was declined, so I tried my own debit card. I entered the correct expiry date (July 2009), but it said "The card has already expired". (I tried this twice, just in case I'd made a mistake.) That's a bit weird, and I'd like to think that any webpages which handle credit cards have been properly tested, but never mind; they list a phone number on that page (0870 888 3699), so I phoned for assistance. I chose the option for sales, and when I spoke to someone he said "Oh, I'll transfer you to sales". The next person I spoke to was a bit confused (asking me whether I was trying to sign up for a free account), and it turned out that she dealt with hosting, so I needed to speak to someone in domain name sales. I would have thought that they'd use the same account page for both, but perhaps not. She transferred me back, and I sat on hold for a while, being told that the expected queue time was less than 5 minutes.
After about 15 minutes, I gave up, and went to GoDaddy instead, where I was able to do the whole thing online in a few minutes. (They're the company I use for SSL certificates as well.)
Anyway, my advice is to avoid UKreg; if this is how inept they are when I'm actually trying to give them money, it doesn't bode well for how they'd react if I needed help later on.