Is Computer Science too competitive for my qualifications

I am thinking about leaving sixth form because I am not enjoying it and might fail in the subject that are irrelevant to a computer science degree. My aim is too study a computer science degree in a RUSSELL GROUP uni and I am considering study BTEC Level 3 Extended Diploma in IT (Software Development) aiming to get a minimum of DDD though I would like D*'s. I am also going to study A Level Maths as a private candidate, to improve my chances of going to a russell group uni. I have checked the requirements and most uni's are ok with the qualification (as long as they have A Level Maths)

My question is, is a Computer Science too competitive for my qualifications, as in are there too many A Level students applying and will go ahead of me. Thats what im worried about, whether there is too many A Level appliciants winning there places over me