At this stage it doesn't need to be impressive 'medicine' work experience - which you will find difficult to secure anyway.
Medicine is about people. Any experience you can get of dealing with people, of communicating, understanding, developing patience, sorting out problems etc etc is VERY valuable, and will score you points on your UCAS statement.
This can be anything like Youth Leader for Brownies etc, any volunteer work at a charity shop, anything you could do at your local church, or via any clubs you are part of. Sometimes small local shops, cafes etc will employ under 16s - its always worth asking. Once you are 16 look at voluntary work placements in aged care homes, special schools or children's nurseries etc - your school might be able to help you organise this to fit around 6th form.