I don't claim to be an expert but I have a forced hot water system (converted at some point from steam) and AFAIK it uses a "standard" thermostat. At least when I upgraded to a Honeywell programmable thermostat 15 years or so ago, I don't remember it being anything unusual. I bought a thermostat at the home improvement store and an electrician who was doing some other work installed it.

Boiler + hot water circulation through radiators is also pretty common in US and will also use a low voltage thermostat. The only exception I can think of is older gas-powered steam boilers which may use a millivolt thermostat (which have the advantage of working when the power is out). (Or electric baseboards...but who can afford to heat that way?)

You must be from a warm part of the U.S.

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.