I've found Telewest's deals to be pretty good, and their reliability is excellent.
Two types of broadband: ADSL and cable.
ADSL is down your standard phone line, and they just send you a modem to connect to your computer. You'll also need filters for your phones that you connect too, but afaik the modem comes with a few anyway.
Cable is with a cable service like NTL or Telewest, and it's usually best financially to get the package of broadband, basic cable TV and phone (�30/month I think is the offer, free weekend calls or something like that), and requires an engineer to come and assess your property then a bit later more round to fit the box in the wall, then they'll fit the cable modem where you want it and lay all the cables.