The most important thing is that you get those grades. Predictions, yes, will be important, but it doesn't mean anything if you don't get the grades. Don't jeopardise that. So if you are doing modular A levels and AS in year 12 then it needs more thought than just A2s in year 13.
As a parent not a student, I would say do it now. Squeeze it in if you can and reflect really carefully on what you see /do. You will have very little time to do a lot extra in year 13, with A levels, applications, interviews, mocks etc. It will be full on.
Plus you will need to take a good few weeks to study for UCAT over the summer.
The volunteering in a care home will take some time to organise. With DBS checks etc. Get the ball rolling now, opportunitites may be limited if you are under 18. So start seeing what you can find now. And start now. Then if it gets too much you can reduce your commitments if needed.
It will be very hard to come to the realisation that you haven't done what you need to and then try and squeeze it in during year 13.
However, can you combine the two and get a part time job in a caring environment somewhere? You can get the necessary opportunity for reflection from public facing employment that isn't in a caring / hospital environment. But if you can double up that might help if you are time squeezed.
Good luck.