I agree, and power negotiation is great.

But things that can use it seem to be expensive. I checked, the chips are fairly cheap; but all the PoE stuff I might use aren't "one per light socket" prices. Plus I'd still like a decent mains relay board that is appropriate to put in a wall - the Shellys' are a good form for this, but I cannot see a similar wired solution (although some Sonos devices get close).

https://www.olimex.com/Products/IoT/ESP32/ESP32-POE/open-sou... is probably the best option

Although I ended up buying Lilygo ESP32 POE from China because Olimex and Brexit don't mix

Hmm, I'm in the EU and still get hit with customs charges from Chinese items e.g. AliExpress stuff. Not sure if there would be British-made equivalent.

But I'd sooner order from a company that is liable e.g. if the IoT device burns down my house.

AliExpress charges VAT now.

If your IoT device burns the house down your circuit breaker failed, or you connected it without proper failsafe measures.

Is it burning it down by being on fire itself, or by turning something on that shouldn't be?

Surely a breaker can only detect over-current or short-to earth/neutral (which would also be an overcurrent I guess).

If the thing just heats up and catches fire, it might not trip the breaker. I meant overheating itself, not erroneously turning things off/on.