If it really is mid-late August, then just for the sake of it regardless I'd push it back to a September plate, if only to not have a number 13 on the car as I just don't think it looks good (I'm daft with number plates like that, nothing superstitious about bad luck or anything!) In either case, the financial difference between a 13 and a 63 plate is going to be negligible in the grand scheme of things.