I'd imagine asking at your local cop shop might put you on the right track.
I think each region in the UK has a regional forensics lab and some considerable daily travelling might be involved if your not near your own.
The other thing that strikes me about it is that much of the stuff they deal with is very confidential and sensitive - I don't know whether they would allow teenagers to work in such environments even if you spend all your time washing test-tubes along with other menial tasks. Crime-scenes might be even more problematical. There's also Health and Safety issues.
Don't let me put you off - call in the copshop as soon as you can. If you don't ask, you won't know!