If you just turn up at a French hospital and demand the kind of treatment that I presume the OP is talking about (e.g., hip replacements or heart bypasses), you won’t get very far. Your EHIC won’t cover it as it’s not a condition that arose while on your trip, so you will need an S2 form (hurry, hurry while the Withdrawal Agreement lasts). Those are issued by the NHS, so I suppose you could make an FoI request to find out how many have been written out this year. I’m guessing it’s not very many. There were a few cases some years back of NHS trusts in Kent sending people to Calais for one or two procedures, but it’s not just something that you can up sticks and claim.