should i self study alvl further maths over summer

I am an international student I told the school I want to do phy bio maths and FM.
but the school has really bad admin and told me ON THE FIRST DAY OF SCHOOL because the time slots of FM clash with physics my only option is to do an irrelevant 4th as level or self study FM.

right now i am considering doing economics/engineering/physics in university and i just realized that for some top unis, FM is not essential but most offers are given to student doing FM , e.g. economics at LSE

Its quite annoying because I have not put any effort into pure maths n still got an A* (predicted) and I know instead of going to pure math lessons my time would be better spent in the FM class.

My situation now is that i screwed up physics (predicted C and I am super determined to get it to A* which is entirely possible if I spend the summer wholely on physics .

I am wondering whether it's realistic to self-study further maths with physics over the summer so that I can do a mock in September and get a predicted grade for further maths UCAS ?

For further maths i am aiming for at least an A.
and i will have to self-study:
pure yr2, FM book 1 (ch1-5), and decision (the whole thing)

thank you for your reply!
for the further maths class in my school we have only done pure yr2, FM book 1 (ch1-5), and decision (the whole thing), and I am now in year12.
right now I have done edexcel pure year1 stat/mech year1.

so i’m not doing the whole fm course over the summer but just 1/4 fm with the whole pure year 2

If it's only 1/4 of the course then it should be okay but you will have to prioritise your other subjects. Also your school is silly because FM and physics are often taken together

I KNOW RIGHT and they literally scammed me to this school and other people who wanted to do physics fm and computer sci were forced to do business instead of physics so they can do FM.