How much a hindrance will my AS FM predicted be towards Cambridge application?

Hi,

My predicted grades are A*A*A B, in which the B is towards AS Further Mathematics, and I would like to apply to Cambridge for Computer Science.

At my sixth form, there is Maths, Maths Plus (students who do A-level Mathematics in one year, taking the examination in Year 13, and AS Further Maths only in Year 13) and Further Maths (students who do A-level Mathematics in one year, taking the examination either at the end of Year 12 or Year 13, and A-level Further Maths only in Year 13).

My initial A-level subjects were Maths, Chemistry, Computer Science and Geography, but by December, I had realised that I wanted to Computer Science at university, specifically at Cambridge.
When looking through entry requirements for Cambridge, I had seen that it is strongly encouraged to undertake A-level Further Mathematics, in which from then, I decided that I wanted to undertake that A-level subject instead of Geography.

However, when I had spoken to the head of department for Mathematics at my sixth form whether it would be possible to do A-level Further Maths, they had said it will not be possible, due to their classes being very ahead in the A-level Maths specification than my Maths class. But had suggested that between December and the start of Year 13, I would have to drop Geography (I did so during January, spring term) and I could join Maths Plus in Year 13 and do AS Further Maths instead whilst also, doing A2 of A-level Mathematics.

For the end of year mocks, for A-level Mathematics, I had achieved a B but due to my performance throughout Year 12 being of an A* student, I was able to be predicted an A* for that subject.

However, for AS Further Mathematics because I had not had any lessons in the subject and had not done an end of year mock on that subject, they had instead used the result of the end of year of A-level Mathematics.

How much would this be a disadvantage for my application? Would it lead to immediate rejection? How much does this predicted grade specifically matter relative to how I perform in my A-levels, the TMUA entrance exam, and the interview?