Colleges and sixth form usually require a B at GCSE in order to do A level maths, there is no other way unless your teacher believes you could have done much better and thought you just had a bad day and may let you off with a C.
If you didn't put the effort in at GCSE, how do you know you will at A level? It is going to be much harder and you will most probably be put off.
And did you even enjoy maths? That will also play a big factor, maths is generally considered to be one of the hardest A levels.
I am not trying to put you off, but you need to know what you are getting yourself into when choosing any A level.