Well i never did well in the gcse (16+) exams, but i got all As in 5 AS exams (17+) in BIO CHEM PHYS MATH AND FURTHER MATH. I am really interested in biology, but at at 17 i doubt i'm recruitable material in that field. How can I wow the interviewer? Where can i find work experience for my age in london in one of the sciences?

I take it you are talking about a interview to go to University. If that is that the case you just need to confident and charming. Make sure you read up on recent discoveries/what the university is researching so you can pretend thats what you want to study there. In biology the lack of experience is not a problem as few people have any experience. Most interviews are quite friendly chats and few are heavy going. I had 4 and they were all more like converstations than interviews. (Im doing environmental biology..at quite a good university...not one of those crappy ones)

just let the interviewer know your keen - give examples like the time when you cut the dog open to see his insides lol

All about passion and interest my friend.

If you are genuinely interested in the field they are interviewing you about then you will naturally digress into areas that show you have knowledge and will demonstrate a keeness to learn.

Also be clear of what you want to get out of the course e.g. at the end of the three years i would like to have a degree that specialises in marine biology etc.