You'd need to speak to your current college to see if they could cater for you. If not, you'd need to look for another sixth form college or FE college which would accept you onto an A level course. Be aware, though, that A levels in Maths, Computer Science and another is a big step up from a BTEC in Games Design, and you'd have to convince any sixth form college that you were capable of working at that level.
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