It seems you are probably not cut out for maths, and even if you were mentally capable (as an A suggests), it reflects poorly on your work ethic to be getting U's in a mock. Either drop maths and stay with three extremely respected A levels (which is still as much if not more than the majority of the country is doing right now), which should be more than enough for most courses, or don't, and take your chances at having to put an 'E' or below grade on your UCAS report and every other application in later life.