I have been with all the networks EXCEPT virgin. And my experience:
Orange
Three
t-mobile
O2
Vodaphone
That was in my order of rating. I am on three now, and the network is fantastic, though the customer services leaves quite a bit to be desired, I like different things about all the networks (except voda, who I hate).
Orange have fantastic customer services, and they are a well run operation, you can't fault their network coverage. (oh and best voicemail)
Three are just SO cheap you can't complain! Great phones and great tariffs. The 3G content is also quite cool.
T-Mobile, well their voicemail is next best to Orange + it is free to check. Their price-plans are the most reasonable of any 2G provider. But their coverage isn't that great/
O2, I hated their voicemail service, and their priceplans were a rip off, but they had good customer support. Their website is very tacky thou.
Vodaphone, I hate vodaphone voicemail, their prices are extortionate, their customer services were very rude when I called them, and the people in the vodaphone stores were next to useless.