What frustrates me is that I cannot get a clear answer from anybody about what happens if, like me, you are an experienced pilot but not current.
That's because you haven't asked. The CAA or BGA cannot issue a blanket "you must sign any document that gets put in front of you" command; that would be stupid. Only the certifier can decide whether he is prepared to have his name forever associated with your documents.
As it turns out, the club CFI, chairman & secretary can all act as certifiers for the conversion. I suggest that you make an appointment to meet up with one of these at your nearest club, explain your predicament & ask if it is worth your while getting an EASA medical, ie if you get a medical, would they be prepared to sign your documents?
However you can't even do that under the current system if the glider is self launching.
Yes you can, with the appropriate PPL or NPPL. It's no different to flying any other aircraft from a farm strip.
ps. you can get an EASA medical from your GP - advice
here.