Carrier Grade NAT
It appears my ISP is testing CG-NAT.
They recently closed their IPV6 trial though.
That does not sound good. I wonder if there's a no-cost opt-out, since they're fundamentally changing your service. While most people won't understand, nor care, the ones that do really do. Or, at least, a low cost option for real address(es)
Right now it's only an opt-in trial so I'm not too worried.
Plusnet are by far the best ISP I've ever dealt with, their customer service is quite frankly astounding. So I'd be very surprised if they started forcing CG-NAT on their users.