I second the wise advice of nexttime and StationtoStation, the most sensible thing for you to work out at the moment is to whether you want to apply for medicine or pursue the biological sciences route. This will not only influence which university you choose but the course of your whole life and career!
However, don't be alarmed, the key at your stage is just to learn as much as possible about both options (feel free to ask me specific questions about medicine at university) and work experience is a great way to do that. Make sure you really pay attention during your hospital work experience, speak to the doctors about their lives and think to yourself 'could I see myself doing this and enjoying it?'
If you do opt for medicine, StationtoStation makes a very good point regarding the voluntary work experience if you are indeed planning to apply this year. Don't forget that you'll need to prepare for the UKCAT (and possibly BMAT) admissions tests if you apply for medicine...
Best of your luck with your decision making, give us a shout with any more questions!
Alex, 4th year UCL medic
6med
P.S. In response to your original post, I love living in London! Plenty of fun things to do and get involved in, excellent hospitals and top quality research happening all over the place, the main downside is it is sooo expensive!