i use ukreg.com for uk names and am generally satisfied, support is very limited by email only and it takes several days to get a reply, but as i've only had one problem that has been ok (it was resolved promptly once they got to it)
on one of the names where i'm just holding the name but using the email addresses there is a sparodic error where mail is duplicated upto 3 times when forwarded to my pop box,
control panel is easy to use, it's straightforward to configure dns records.
i've used ukreg and 123reg but i got so fed up with problems i now have my own nominet tag and my own opensrs account.
typical ukreg problems include failure to set nameservers correctly and failure to register .uk domains. if you get a reply from ukreg support within a week, you can consider yourself very lucky indeed. i've known them to take over a month to reply to support emails. they definitely cost me business due to poor service and poor support.
i've been told easily have good support services, but i've never used them myself so cant say any more on them.