Wow, just skimmed this chat. I'm only a year 13 but I think if you want to apply to a great university, FM you should definitely do (and should definitely want to). But, a lot of universities don't require FM and this is fine too - you can still get a Maths degree and you may even cope without FM because you may be more mathematically mature at undergraduate age. But OP is still year 11, unless I misunderstood, so they've got plenty of time to push it to 8/9 and do well at A Level (maybe with better revision or private tutors) and plenty of time to think about whether Maths at uni is for them and what universities wouldn't require FM (most don't but will lower boundaries if you do).
I do think that grade 8 requirement for FM is fairly sensible - it's a demanding course with the strongest cohort and not hugely low boundaries considering. But, some people could still do well with a 7 because a 7 could be because they've had holes in teaching/knowledge/lack of practice, etc which could all be cured in sixth form. I know my friends got 8s/9s and some are finding it very hard, some are finding it manageable. One of my friends got a 7 and he's doing well right now by working consistently (although resat the year due to family issues first year around).
TL;DR, OP has a lot of time to think all of this through. FM is not a necessity but someone who wants to do a Maths degree, I'd think, would want to do more Maths which includes FM (most of the time this is true but there are people who improve a lot during uni so it's not definitely true).