I think Gandalfs advice on the subject might be the most relevant. See what the people who live about you use, how good their service is and what the offer is.
The reason I say that is it seems to me to greatly depend on the area you live in, some suppliers are really poor in area A but good in area B and vice versa.
I was with Tiscali but after about £20 of phone calls to their so called help line and the pack of lies they eventually told me as well as being knocked off for a fortnight I really wouldn't recommend them.
I with plus now and have found the connection speed consistant and stable. I get about 3-3.5 meg connection on my up to 8 meg line and that seems reasonable to me as I am a fair distance from the exchange.
Customer service is good and is UK based.
Only thing with them is you have to setup your connection manually as they don't do a disk, wasn't a problem for me but could be for some.
Heres their site click here