If you're constantly filling your bandwidth up and down then they can legitimately claim that you are adversely affecting service to other users. I expect them to clamp down on the very heavy P2P users, because upstream causes more trouble than down. I doubt they'll hassle people who have a habit of downloading gigs of "Linux ISOs" a day from usenet,for example, as they won't really stand out in the crowd of problem users.
FWIW, I've been with NTL for 6 years now, always top-tier and regularly breaking the limits (but not massively). I've never been contacted and when you see stats from ISPs showing how their usage patterns break down, it's obvious why -- the problem guys are stuffing their pipes 24/7, everyone else is nowhere near.