MATHS GENIE! Best website EVER. each GCSE topic, a video for each, questions for each, answers for each. Do all the topics you're bad at then do a past paper in timed conditions. Mark it, then for the topics you got wrong, redo the maths genie. Got me a very high 9. Takes very very little time. You can have a whole topic done in 1 hour.
Just a note though, maths a level is incredibly difficult. If you're struggling to get an A, it might not be the best choice. I got a 9 and breezed through GCSE and struggled a lot on the A-level. But if there's another reason and you do like maths and know you'll be able to pull it off, more power to you xx