I have allways found 123-reg very good for .co.uk names, they are allmost allways registered instantly, however I have had a couple of problems, like someone said earlier, if the whois hasnt changed within about half an hour, you know theyve messed up, but you know they wont do anything untill the 48 hours it can take up to to propogate...
And their email support is awfull.
I regged a .co.uk that apeared in my control panel, but they didnt actually register, and it took them a week and a half to respond to my emails, asking me to email them back again with whois data so they can register it manuallly, another week later I had my domain.
Thankfully this has only hapened once.
On the up-side they are very cheap for .co.uk's - free transfer in and out and free accoutn push :-/