how vital is comp sci for a comp sci uni course

i recently finished my gcses and was a mark away from choosing comp sci at a level which was devastating, after a remark i was still smack bang where i was, my initial choices were: maths economics and computer science but i have to choose any other subject except physics or chemistry instead of computer science which was preferred, anyway i was wondering if i can still do comp sci at uni and if so what would be the best a level to replace it minus chem or physics? and also is comp sci at a level required to do comp sci at uni?

No. Maths is more necessary. Look at entry requirements for the unis you're thinking about. Eg UCL only requires A* in maths for Comp Sci Bsc. Most unis will teach comp sci from basic knowledge.
I don't think unis have a preference between physics or chemistry for that course but you can email or phone up departments to check this. Personally I think physics is easier than chemistry.

i can’t choose physics or chem ffs LOL i’ve been told to do two other subjects i like and i’m good at politics sociology and economics

my bad, I misread. In that case, since they're not major subjects for comp sci, I'd look at the grades of previous years in your school for each subject because the grades are what's going to matter not the subject here. If your teachers are bad you can easily struggle. Ask people in the year above etc.

oh i see so choose subjects which i enjoy/know that will get high grades in my specific college?

No. Maths is more necessary. Look at entry requirements for the unis you're thinking about. Eg UCL only requires A* in maths for Comp Sci Bsc. Most unis will teach comp sci from basic knowledge.
I don't think unis have a preference between physics or chemistry for that course but you can email or phone up departments to check this. Personally I think physics is easier than chemistry.