Can you explain what you mean by your phrase

"However, any installer
specifying a 24kW combi sounds like the sort of cheapskate that
wouldn't
even know what a boiler interlock was. "

Arent you making an assumption on the size of the house the boiler is
fitted to?

There is obvioulsy some form of stat in the boiler itself as it does
turn on/off and does seem to maintain a level temp.