I think we did the same exam, I'm in year 11 and I'm getting my maths results at the same time.
In regards to the grade required for FM, a lot of the sixth forms I have looked in to have an absolute minimum of a B grade (but this is really stretching it in my opinion) at GCSE maths. There [technically] isn't a specific grade you need at GCSE in order to do well in FM, as long as you're determined and motivated in your AS/A levels you can do really well.
This is coming from someone who hasn't actually done their A levels, I'm in the same boat as you, however I've also had to make the decision of picking FM or not (with a lot of help for TSR!) and I decided to do it. I've already purchased revision guides for my A levels (I can recommend CGP guides but it depends on what exam board you're doing, find out from where you're applying) to get a head start and it's clear that to do well in AS/A level maths or FM you will need a very strong understanding on the A* topics like rearranging complex formulas and transformation of functions for example.
If you're worried about not getting the sufficient grade and having to retake, it is entirely possible to self teach yourself the topics that haven't been covered by your maths teachers yet. I sat the maths GCSE a few months early like you and therefore a lot of the key A/A* topics weren't covered, however I self-taught myself them with the help of a CD program called MathsWatch. I honestly could not put across how useful it is, it has lessons on every topic from grade D to A* for GCSE maths and the person shows you every step of pretty much every question that can be asked in a GCSE exam. I think you can't purchase MathsWatch yourself, your school needs to buy them in bulk for their students. But if you can, put some research into it and try to convince your school about how beneficial it would be for both you and the establishment's percentages of A*-C.
I'm sorry if I've gone way off-topic, I'm just trying to help as much as I can. Best of luck for your GCSEs.