Hey,
It's great to know that you are aiming to do somthing like this! I am 16 right now and have been volunteering at a local Rescue Squad (it has a payed day shift and night time volunteering shift) and go there once every six days. Right now, because it's the school year, I can only stay from 6-11 pm (5 hours) but normally there are 12, 18, 24 hour shifts.
Right now, over the summer, I got certified as an EMT (sure you know what that is). You might be an EMT or something equivalent right now as well. From my experiences, I can tell you that if you really enjoy something like this, you will enjoy working hard at it. I will tell you that it takes time to get used to it. It's so much different than what you learn in the classroom.
If you are aiming to do something in the medical field, I would suggest volunteering at some other medical places like a Red Cross, hospital, and definitely a rescue squad (don't know if UK has them.)
If you have any questions to ask about volunteering at a rescue squad, feel free to ask me.
Thanks,
Shashdev