I don't know the BT limits (don't use BT), but when I was looking to change ISP a few months ago, most had a "value package" of about 3 gig a month. Exceeding that either meant the system slowed right down (no TV) or was shut off altogether unless you paid for more.
I suspect BT is 10 gig a month, based on what a pal of mine says.
I chose an ISP with 24 (or is it 25?) gig a month, and don't get anywhere near that, even when the family comes to visit. Most months I'm around 10 to 12 gig. Watching television does rack it up pretty quickly, but I don't do that: we have a television downstairs and I have a TV dongle in the PC - although I rarely use it.