Are there any colleges that let you pay for singular A level exams?

I got my A levels last year. DDC. Not through being stupid; more that I was miserable, things weren't going so good and I basically had a lot more on my mind than my upcoming exams. I got into a mediocre uni anyway, through clearing, but when I got there I sort of realised that I was capable of a lot more.

So here we are. I plan to apply this September for 2012 entry, but realised that I could probably do with getting better A levels.

Is there any way I can get new A levels before this September? I know there are summer programs, but I don't have that much money. Besides, I am very good at teaching myself, despite evidence to the contrary.

Ideally I'd like to be able to pay a college to let me sit exams in a couple months time, and get my results with all the other A level students. Is this possible?