United Kingdom

Most modern boilers can be activated by a simple relay (it's how the thermostat calls for heat).

A basic switch plumbed into hass can be set up as a thermostat entity, that isn't too horrible to use.
And you can add a physical thermostat capable of sending values locally to hass if people want to be able to spin a dial on the wall.

To be honest, I do a lot of my automation invisibly: The target temperature is automated, the only physical button is a "30 minutes heat" one I installed.