What are your GCSE grades?
You don't necessarily have to run a medical society. Is there one at your school? Are you participating in it? Have you made any contributions to it (e.g. done research and given a presentation on a topic of interest)?
It's good that you're doing volunteering and have done work experience. What's important though is what you can say about it. What have you learned from it? How has is it impacted your thinking/perspective? Has it caused you to develop an interest in x, ...which has led to y?
If there are STEM challenges e.g. biology/chemistry olympiad type, it could be useful to participate. Don't need to "win" or get the top award, just taking part shows your interest.
What's left is the UCAT - this will likely be the deciding factor for whether you'll have a strong application. As for your current situation, you have a solid foundation to go off.
For now, make sure you prioritise any AS/end of year exams, followed by UCAT preparation.
Good luck!