Hi again, and thanks for all the replies. I did some more digging and found a copy of 'Border Measures Q&A - Aviation Sector' issued on 5th June. I'm afraid as I have less than 10 posts on PPRUNE I can't share the url, but I found it on the BBGA website.
On page 12 under the section headed 'Will the below crew scenarios be exempt, or will they have to go into 14-day quarantine, one of the scenarios is:
d) Crew flying into England commercially to carry out simulator training at a UK approved centre?
The answer, which was a surprise, was Yes. "Crew who travel to England for the purposes of attending training in England are considered to be travelling in the course of their work and therefore they are exempt."
So according to that rationale, it would appear that travelling to England to renew a medical would also be considered 'in the course of their work', on the basis that a medical renewal is just as necessary (and unavailable locally) as sim training.
I'm assuming it's all change from Monday anyway but unfortunately I'm arriving on Sunday. Thanks again everyone for your thoughts and ideas.