An update, in case anyone with a pre-April 2015 visa is wondering:
I was able to get my routine cervical exam at the local, not-my-GP service just by providing my reference letter. I was armed and ready with passport (containing visa) and a printout saying that the IHS surcharge was for visas after a certain date, but didn't end up needing them. That doesn't mean that other hospitals/clinics/GP surgeries won't be confused, but this lot either didn't know or didn't care, which suited me just fine!