For most MPharm you will need Chemistry and a second science at A level.
Some Unis may accept a BTEC Extended Diploma in Applied Science - but you will still need hefty Chem content.
Check a range of Uni course pages for their entry requirements.
But the other thing you really do have to think about is :
"If I'm not actually that good at science, will I actually like doing a 4 year degree, and a career, that relies on knowledge of, and enthusiasm for, science."