Usually colleges advise you that if you want do a computer related course at college, you need either a GCSE in IT or Computer Science, because not all schools offer computer science. I'm surprised college won't accept you on just a normal IT GCSE, unless you aren't doing that either.
You could also go for a Computer Science BTEC instead of doing it in an A Level because the entry requirements are usually more lenient on you and only ask for the 6 GCSES at grade C and above for a level 3 course. They just expect you to have an interest in computer science.
When my school first offered it to year 9s I was the first year group to be offered. The year 10s at that point were also offered the course too, but they would have to stay behind after school on certain days to complete the course. Many of these year 10s coded at home for a hobby and already knew a fair bit and were really dedicated. They all completed the coursework at home and I know a few managed As and A*s. If you can prove you already know a lot about computer science they may allow you to do the coursework and then sit the exam.