My mum is a GP, and one of my friends did work experience with her.
The receptionist just asked each patient before they went in if they minded having a work experience kid in there, most of them didn't mind, a few said no, but you have to expect that.
Its good especially if you go to a practice that has a lot of mums with kids, cos the mums dont mind so much, whereas adults dont really want people sitting in on their consults.
Maybe ask your doctor, or if you have any friends with doctor parents they might help out.