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Do A Level Chemistry in your gap year (budget for £1500 for exams + whatever your course fees are if you decide to take the course)
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Try to convince your teachers to take on Chemistry on top of your other A Levels (either drop a subject or do it as a 4th subject, if they don't mind you staying on 6th form until you're 19)
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Do a foundation year that would specifically lead you into pharmacology (costs £9250, unlikely be able to switch universities afterwards)
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Do an Access course that has both biology and chemistry at 15 credits each (costs £3500 roughly)