As my 5th choice I am thinking of biology/biochemistry at imperial with AAB entry requirements, however do you think I would get and offer or have a good chance of getting one with a medical PS???
I got an offer from imperial for biology with a medical PS last year. So long as you're predicted high enough (assuming you are cause you're applying for medicine!) I see no reason why you'd be disadvantaged by a medical PS, I wasn't!
I've seen many people with medicine PSs get offers from Imperial for Biochem and Biomed, I would assume it would be the same with Biology or any other kind of related science. They might ask for an alternative PS, but that doesn't always happen.