As far as work experience goes I think you need to look further than simply volunteering in a GP/Hospital. Really, what you need is work experience which shows you have an aptitude for Medicine. This can be in a variety of different ways. For example, here are some which some people in my year have done:
- Care home volunteering. This is fairly popular as it enables you to get face to face experience with patients in a way that volunteering at, say, a clinic doesn't allow you due to confidentiality laws.
- Assisting with SEN children in school. This is useful because it shows a position of responsibility as well as dealing with children with (sometimes fairly severe) special educational needs and behavioural problems.
- Lifeguard training. This has put them in a role of public responsibility, as well as including a mandatory first aid training course.
Basically, use your imagination - there are a lot more relevant places you can gain work experience for Medicine than simply a hospital, with pretty much anything that shows you acting in a caring role towards others being a big bonus.
I would do these during the summer of GCSE and AS though - Medicine also expects you to perform well academically and you shouldn't underestimate the jump in workload required to perform well at A Level.