MixedBeauty: You can totally go back to college at any age in the UK. Maybe not a structured 6th form (if you want to do A-levels) but college for sure.
My friend did an Econ degree and afterwards decided she wanted to do Psychology and Neuroscience but didn't have the right A-levels. So she went back to college and did Chemistry and Biology in 1 year. Whether you want to do A-levels or any other courses (NVQ, BTech, whatever) it's easy to go back to college. There might be some minimum requirements such as gcse passes but no biggie. At the end of the day, what matters is your determination to do well, even if the college isn't amazing.