Zen are very good customer-service wise, but normally not the cheapest. They do a lot of business products too so have good support (all in the UK).
Do you have sky tv? If so, then I'd go with them as they do good bundle prices (was paying £10/month uncapped with the tv and phone package). Used to have sky broadband but when we moved I cancelled and went with BT Infinity so I could get the BT Sport channels for free
