I'm currently with Sky, never had any major issues, no real down time (other than the naf router they provide you with - too lazy to upgrade). Copes with all my uploading/downloading through FTP and constant internet usage fairly well. But we're on the Â£17.99 package because we live in the sticks, so only one choice really!
There's a company called LonsdaleNET (http://lonsdalenet.co.uk/) that work locally and they provide up to 60mbps with a guaranteed 10mbps on installation (otherwise they won't install). It uses LoS (Line of Sight) wireless transmission. It looks very good, and I've heard great reviews around the town, but they're a little expensive on installation too.
I have used some crap ones in the past, Toucan (now part of TalkTalk or something, can't remember!). I came from Rochdale (where Zen started) and they've had a terrible past, really bad service and terrible connection. From what I hear they're still not much better. But like anywhere you'll get great and terrible reviews from different people!