declining grades?

I, like many people here, got my AS results back on Thursday. I achieved As in Sociology, Politics and English lit, and a B in History.
That B was rather unexpected, I was predicted and well on track for an A- the bright side is that I was only 5 UMS away from an A and the whole class's grades were pretty wacky.
Now, my teacher may be asking for a whole class remark, but even if he doesn't I want to retake a unit to get the A.

I want to apply to Oxford, so I don't want to have to put down the B when I'm going to be retaking it. I've heard that you can ask the school to decline a grade and not get it certificated until after a retake. How would I go about asking them to do this? Is it okay to do it when we start back in September or will I have to get into contact with the exams office before then?

:confused: