I'm kind of in a similar situation as I did very little studying and no revision and came out with a C 10 years ago. However, I now need a B to get on to my chosen degree(like you it pisses me off to a certain extent as I know I could have done miles better).
I've attended a GCSE class for 2 hours a week since November and have got to grade A standard on past papers quite easily. If you slacked and got a B you should have no trouble brushing up to get to A/A* standard at GCSE before you do an A-level.