I know with things like Tado and EvoHome the TRV heads tell the thermostat/hub what heat they require then the thermostat tells the boiler, however not sure if that can be regarded as independent? And also it means boiler must be OpenTherm enabled, Worcester bosch is not OpenTherm and it has a single thermostat which does not as far as I can tell connect to the TRV heads.
Hive is an odd one, does not have OpenTherm it simply switches off/on, when the TRV's need heat it sends a "heat on demand" signal that causes the wall thermostat to switch on, even if the room it is in is over the temperature set, so the set temperature on the Hive wall thermostat is really a fall back temperature and if set low then each room would be independent and the boiler would work on the return water temperature until all Hive TRV's are satisfied, as to if that would be considered independent in law is another question.
Nest is OpenTherm but has an odd connection to TRV's, it has a follow command, so if you alter the Nest dial it also alters all TRV heads set to follow, which to my mind is not independent control.
Using programmable TRV heads the rooms are independently controlled for both time and temperature as long as a wall thermostat is not fitted, as soon as you fit a wall thermostat you lose in theory the independent control, in the main the wall thermostat is only to stop cycling of the boiler, but it needs to be set to a temperature that will allow that, if used to control room and the TRV in that room is either set high or missing, it is doing a different job, so not independent.
I see no problem in complying with the law, non is there a problem in controlling the central heating, however there does seem to be a problem in economically controlling the heating and at same time complying with the law using the controls available at the moment. Unless there is a control I have missed?
I think the modulating control uses voltage to set level of modulation, so if the range was 48 volt and you have two thermostats in series both able to send out 24 volt then in theory they could work together, however they would need to be designed to work in series, and this is the point I can't find a ready made system that allows use of two modulating wall thermostats.