If BT are upgrading your area to broadband you have many better options than a 5gb a month limit. I'm on a 2mb connection with 3 pc's and on a bad day top 3gb. I suggest you look around before jumping in with BT. Also if your phonline is with NTL I hear you could get AOL without a change of phone provider and they are unlimited but costly compared to some.
Also a lot of folk do not like them because of all the gear that comes with them
Max