These are room thermostats and they also can be set as to what time as well, the one shown Bluetooth cost me £15 each, without Bluetooth £10 each, Hive
are £54 each however there is nothing to say you must use Hive TRV linked to the wall thermostat as said with some thought you can have Hive in some rooms and cheap in other rooms.
Once the head is swapped to electronic they are motorised zone valves, the difference is how they tell the boiler what to do, with the old style motorised valve there is a micro switch in the valve which connects to boiler, with new it sends a signal to the wall thermostat which connects to a boiler.
The big difference is old zone valve switches on/off, new TRV head switches up/down so it has a lot of advantages.
1) It allows the boiler to modulate as it is designed to do, so uses less fuel.
2) It does not completely switch off, but changes the set temperature, so with Eco and Comfort setting one can heat a room from pre-set lower limit to pre-set upper limit rather than allow it to get stone cold, so ready for use faster.
3) Every room is its own zone, so if a bedroom is used as an office or craft room that room can be warmed without also heating bedrooms.
4) The cheap TRV shown has open window detect, so in the kitchen when the outside door is open to unload car, it auto turns the radiator off, then reheats room once the door is closed.
5) As well as phone, one can simply push a button to flick between Eco and Comfort or press dial for boost or turn dial for non programmed temperature (that's the cheap one, I don't have Hive)
I am very pleased with the 9 electronic TRV heads in my house, 4 expensive Energenie and 5 cheap eQ-3, but I have Nest and unfortunately when Nest was taken over by Google they removed support for Energenie so they no longer link to wall thermostat. Hive uses a demand for heat, so when the TRV wants heat the wall thermostat turns on the boiler even if the wall thermostat is warm enough, in other words the TRV controls room temperature, the wall thermostat is a hub relaying to boiler what is required.
OK with your boiler Nest would connect to OpenTherm so would reduce the temperature of circulating water as required, which would also mean zone valves would not work, as can't connect two OpenTherm thermostats to same boiler, as to which is best, not so sure, EvoHome has a thermostat which is a panel where you can set each room independent but OpenTherm is add on extra, Tado also has a system, not sure how that one works.
Back 10 years ago we would fit motorised valves as the boilers had no ebus option, and also would not fit TRV's in same room as the wall thermostat, but things have moved on, even in 2018 there was no Hive programmable TRV head, which is one reason I went for Nest not Hive, but things move on.
If you had the money there is nothing wrong with fitting iVector fan assisted radiators in every room, and you would not need a by-pass valve, or a motorised valve, the L1 statement is one persons interpretation of what is required, as with Part P electrical regulations the actual law does not say any such thing, it is written so it is flexible enough to cover different methods to get same result.
