I use 123-reg | Domain names | Domain name registration - but not because they are good, it is just force of habbit I've been using them for well over 10 years and its easier to look after a lot of domain names in the same place.
123reg have started insisting on continuous payment authority (CPA) for new accounts, which is a *very bad thing*:
Regular Payments off a Credit Card Discussion - MoneySavingExpert.com Forums
Dunno about existing customers.