My friend took up chemistry over summer (y12 to y13) as a 4th a level. He self learnt all the 1st year content over the summer. The teachers agreed that if he got a C or higher in a chem AS paper mock first day of Y13, they’d put him into Y13 lessons. He ended up getting a B. So I’d advise going to your head of year and head of departments to see if you can do something similar. Obviously instead of taking up a whole new subject you’d be substituting one for another.