Not as bad you make out. If there are problems with the future availability of AVGAS 100LL, it will not be the end of piston-engined flying. AVGAS 91UL has been around for a decade or so, and is suitable for most GA engines. If you look at the website of the UK's Light Aircraft Association, you will find a long list of engines that can use that fuel. Yes, it's a UK specific approval.
I understand that the EASA position is that they are OK with it if it's approved by the engine manufacturer. But - don't take that comment as authoritative.