I am applying to study maths next year without FM and got all 5 offers (Southampton, Exeter, Surrey, Nottingham and Loughborough). You will only be slightly restricted in terms of where you apply -Durham Warwick Bath Bristol need FM as well as oxbridge and UCL.
At every open day i went to for my options, i was told that it is fine not to have FM as they cover everything as if nobody had done it.
Alternatively you can teach yourself some of the units, I am currently going through FP1, D1 +D2 just because i want to but am finding them a lot easier than the A2 maths modules. So you could easily just buy a textbook to look through in the summer before you leave for uni. Hope this helps