In my case, the boiler will switch on/off, it can't do anything else. With a boiler that can modulate, then the aim is to control it with the TRV heads, however if the heads are not linked to a hub, there is a problem, as clearly once the weather warms up, the boiler would sit there cycling on/off as that is the only way it has to know if all rooms are satisfied.
So we should fit a wall thermostat in a lower floor room (as heat raises) which is not kept too warm (as don't want it to fire up in the morning on a day which is likely to be a warm day) and where there is no alternative heating (this includes the sun through windows) and no outside doors.
In the main, the room simply does not exist. As next best is the hall, but it needs some careful balancing. We have three controls, let's deal with the first two, the lock shield valve (controls speed radiator heat up at) and the TRV, the TRV takes time to adjust, so if the lock shield is wide open, before the TRV can close the radiator is stinking hot, so I set the TRV head to say 18ºC and if the temperature exceeds 18ºC I close the lock shield valve a bit. Easy with an electronic head

I can see on the PC or Phone, both current and target, but with a mechanical head, it is a bit of guess work with *123456 what 18ºC is, likely around 2.2 to 2.5, but also need to measure the return air to radiator, we are told to use a differential thermometer to set to lock shield at about 15ºC difference, one most householders don't have a differential thermometer, and two still left trying to work out what is 18ºC on the TRV, so until the lock shield is set, need to use an electronic TRV head.
So if the wall thermostat is now set to 19ºC, then the boiler will only turn off on warm days. Once all set up, you could return to the mechanical TRV head, so you could move the head around room to room setting the lock shield valves.
However, I don't want the same temperature 24/7, so I need an electronic TRV head in every room, so I can set time as well as temperature

even the cheapest, here shown eQ-3 which cost in 2019 £15 each, I can set what temperature and when, OK Bluetooth to one phone only, but it works, more expensive types

can show a full report on how they did, and can be accessed by multi devices, even voice commands, in my house hey google turn living room to 22ºC will turn two TRV heads and the wall thermostat to 22ºC. Showing Kasa TRV head. Use Wiser TRV heads, and they tell the hub when to turn on the boiler. I have a Wiser wall thermostat, also two other, but in theory with Wiser no need for wall thermostats the TRV heads and hub will do it all.
Since my TRV heads are not linked, if the wall thermostat turns the boiler off, clearly the rooms will cool what ever the TRV head does.