Hi Anth,
I'm with O2 for broadband. You do get a discount if you're already an O2 customer (£2 or something a month) but unfortunately i wasn't.
The prices are good as well. I got a 17mb line for £15 a month. I've checked the speed and it is indeed 17mb. If you don't particularly need broadband i wouldn't recommend getting it as you do still have to pay a £10 a month ransom to BT for the line rental.
Having said that, the service i've received so far from O2 has been great, and i couldn't recommend them highly enough.