The Heat Link has a big button on which lets you turn the heating on and off, overriding what the Nest is telling it. That's not as economical as a regular thermostat, but it means you won't freeze: https://nest.com/uk/support/article/How-do-I-turn-the-heat-o...
Forced hot water systems are very common in the colder parts, and they use the same thermostat wiring as forced air. Some parts of the U.S. even use diesel fuel for heat (it's called #2 heating oil, but it's really just diesel fuel with pink dye added so they can fine you for using it as a vehicle fuel...it's taxed advantageously).
Also, boiler-radiator systems can absolutely work with standard thermostats. The classic design has no electrical control at all, just bimetal valves (like car thermostats) to regulate the flow of heated water. But the modern revision uses the same kind of electrically controlled valves (24VAC, IIRC) that forced hot water does.
All low-voltage thermostats with standardized wiring layouts.
I didn't know the UK ran high voltage to ordinary thermostats though, that seems crazy if it's just a control circuit. Presumably the full mains voltage?
Old U.S. "radiant" (electrical, non-hydronic) heat ran 220/240 VAC to big rheostats in each room, but that was actually the supply to the heating elements. It feels like something that wouldn't meet code today, but it used to be popular in some parts of the country where electricity was inexpensive.
http://www.cnet.com/news/nest-thermostat-finally-arrives-in-...
https://nest.com/blog/2014/04/02/the-uk-just-got-a-little-mo...
The US version won't work with line voltage systems. It looks like the UK version only works with line voltage systems.
This also explains why in the US changing a thermostat is something pretty much anyone can do but the UK version requires a Nest Pro to hook up.
Nest US: "It's easy to install the Nest Thermostat. Everything you need comes in the box"
Nest UK: "A Nest Pro installer should install your Nest Thermostat. They will identify cables from the boiler, zone valve or junction box to connect to the Heat Link."