Dropping a subject?

I achieved a B in AS maths (one off an A) and a very high A in psychology (still need the exact UMS tho) and would like to drop maths to an AS and take psych to a2. Is this possible since my school doesnt offer a2 psych so I would have to self study it. My past academic grades are very good so would they trust me to self study it? Will be applying to medicine this year hopefully