A-Level Further Maths Dilemma

I have a small problem and I really need some guidance. I want to study a meng in Aeronautical Engineering at a university or through a degree apprentechip and im in Year 12. However I got a 7 in my GCSE Maths which meant although I applied for Further Maths and got an offer for it from my school I wasn’t later given that offer because I didn’t quite get an 8. I have talked to my school since enrolment say up until now trying to convince them to let me do Further Maths despite my 7 but there problem is that they have a long waiting list even with people who got 8 9s who haven’t been let onto it. So they did propose another idea that if I really wanted to I could self teach it, which would mean I would have to cover all of A level maths in year 12. Then they said they would let me mock the further maths students sit at the end of Year 12 and they could mark it however because I didn’t learn it with them they may not be able to give me a predicted grade. So I need help here, should I give up on my Imperial or Cambridge Aeronaufical Engineering hopes and just focus on my three a levels my Maths, Physics and DT and getting into a Russel group university or should I try to finish A level maths in Year 12 and get my tutor to give me an assessment and a predicted grade for “Further Maths” as I think At A-Level this can be done? Or should I just give up on getting into Imperial or Cambridge? Because another thing is apparently if your school offers Further Maths and you don’t do it then it looks bad to Imperial and Cambridge, even though the case here is that I want to do it but my school won’t let me.
Thank you,
Elliot