Just checked on the BT usage calculator. 80 hours of surfing will use approx 4.4gb per week. 10 hours of radio will use 1gb. If you go to BT dot com click here click on broadband, then more info on any of the packages, then usage calculator, you can have a play with all the options to give you an idea.
I do think though that the calculator is a little on the pessimistic side as I surf for well over 120 hours each week and listen to radio for about 10 hours a week + all my email and downloading of updates etc, not yet gone over 2 gb with my ISP in 2 years and my ISP is unlimited.