The likes of Boots Pharmacy will probably take you on for work experience - one of my friends did work experience there when she was 15. You won't be allowed in the prescriptions area but you should be allowed to work in the over-the-counter drug sales area and you'll probably just shadow another worker or help stack things. So, really, you won't be doing much. My advice is to look around for Medicine summer schools for Year 11-12 students. They're free, and lots of universities offer them over the summer. They're quite competitive, but if you've got the grades then it shouldn't be a problem getting in.