Hi mate, I did medicine at a russel group university. Considering you're in year 11, just focus on your GCSEs at the moment. You need mostly A*/A grades for most russel groups. Top ones like oxbridge will find it hard to accept you if you get anything below an A in them however.
That being said, medicine is extremely competitive everywhere and will require high grades, extracurriculars, good references from teachers where ever you apply. The university you go to doesn't matter at all for medicine so base your decision off what course structure and city you prefer instead.
Extracurriculars can be anything you like as long as you can link it back to medicine. I volunteered at a hospital for 6 months every Saturday but I know people who did a week of volunteering in a local hospice for example. Try to also do stuff that shows you're engaging with activities outside of academics such as programs your sixth form might be putting up.
Another thing is to smash the UCAT, I did a month of prep which was enough but doing more than that wouldn't hurt.