RE the discussion on 3 or 4 A levels, I really don't think it matters at all if you are a top candidate. Imperial aren't going to turn away a guy with AAA at AS, and A*A*A* predictions and a great personal statement + reference for a guy who also does further maths but got all B grades. Obviously if the second guy got 4 A's with A*A*A*A* predictions, he'd win. But don't worry.