I was 18 and I applied to a sixth form college for A-levels, however It didn't work out for me. They did accept me but I hated the environment! It felt just like secondary school and I had no freedom, horrible.
I am now going straight from GCSE'S to doing a degree at open university. If you don't have any higher qualifications then the government will give you a student loan for a degree. (This is a good option if you think you could handle the jump between GCSE's and a degree)