Heading for a B in AS maths.

Ok, I know trying to get into cambridge for maths is very very hard, so this is a bit ambitious of me. Figured I'd ask for advice on here. If I get a B in further maths AS, would I be better off rejecting the grade, and say I'm doing the whole course in one year, or should I face the facts like a man?
I really like Maths, but at the end of the day, that's not enough! I don't even know why I'm even giving this a try, someone put me straight please!

Edit : Sorry, the title should say FURTHER maths.

worry about it after results day. everyone feels like they've done worse than they actually have...

Well, the deal is we do Further Maths in two lessons a week. The way we're *supposed* to get an A is to do really well (High 90's) in D1, and reasonably well in S2( low 90's). This means we can slip up on P4, as it's really quite hard in comparison, especially if you haven't done P2 or P3 ( that's just the way our school does it). Unfortunately, I ran out of time in the D1 exam, missing out on a lot of marks. I'm about 20 UMS marks below my target, and this could cause a lot of problems.