A room thermostat.what am I missing here?
The boiler heats the water to a set temperature and will then switch on/off at intervals to keep it at that temperature. It has no way of knowing how many radiator valves are open, or what the air temperature in the building is.
If the valves are all closed, it will continue to cycle on and off to heat the water which will just be pumped through the bypass, wasting gas and causing unnecessary wear to the boiler components.
TRVs with electronics will control the boiler as well, so if all are closed, the boiler will be switched off until heat is required again, so the boiler will only be on when heat is actually required.
Better systems will have a central controller which will activate the boiler based on how quickly the air temperature changes, how many valves require heat, how long it has been since the boiler was last activated, etc.
If you currently have no room thermostat, installing a £10 one will immediately save money.
The Zwave or similar systems may well save more, but will also cost a substantial amount to buy and install.