Private A levels

I'm currently doing 3 a levels at my school (6th form) however I'm strongly considering doing 4 because it will be much better for my future I think?? Is it possible for me to do a private a level?? I've seen a few things about them online but I'm not sure if I can qualify or what I have to do?? The a level I want to do privately is further maths, is it possible for me to do it now and how would I go about doing that?? Because its been well over a month since school started and the option to sign up for it at my school is closed now.
Thank you 💜

What degree are you thinking of? Why do you think you need 4 when no uni requires it?

I want to go into computer science preferably at Imperial and my teacher said I would be very unlikely to get in with only 3 a levels and no further maths (Currently I'm doing maths, computer science, chemistry) because Imperial is already a very tough uni to get into and on top of that computer science is a very popular subject.

https://www.imperial.ac.uk/study/courses/undergraduate/computing-beng/

Read this - you don't need four A levels ...

I don't understand why the school are saying this but not letting you start ... it's seems strange to me. What grade did you get for GCSE Maths?

I would honestly look elsewhere as Imperial is not a good place to study ... student support is poor

I got a 9 in maths gcse and 8 in further maths..and yeah I don't understand why my teacher would tell me that and then proceed to tell me I can't do the a level at school..but he said something about how if a uni takes 300 students a year and 270 of those students got As and A*s while doing 4 a levels then unless I'm one of them i have very slim chance of getting in. Apparently one of his close friends works at a top uni and told him they always prefer students with 4 a levels and/or further maths 😭