Many of them have this 'fair usage' kopout, but no one can define it. No one will dare quote a number until you have used xxx gb!
I've looked at this chart and played around with it. You might get an idea of your usage, then call your provider to see if it makes sense.
Broadband Download Usage / Bandwidth Limit Calculator
By the way I'm on O2 and am always downloading movies, songs, chatting ob Skype, watching TV catch up sending large files and don't seem to have any problems. yet!