Nest and Tado can be on/off or OpenTherm which are you looking at?
Nest was able to link to Energenie TRV heads, however when I tried I found it worked reverse to expected, the Nest controlled the TRV not the TRV controlling Nest.
Using the Energenie app when I altered Nest the TRV's followed, however when I turned Nest dial or when Nest was set for a timed change, I found the TRV failed to follow.
I rang up Energenie and was told Nest had removed support, so the TRV's no longer link to Nest they are just set with same schedule.
It seems in USA Nest has released temperature sensors so it can monitor the temperatures in 4 rooms, but these have not been released in the UK and the Nest units are different UK to USA so seems unlikely they will work.
In fact Nest Gen 3 has been a disappointment, main reason for Nest is only two wires between main house and flat where boiler is, and they change colour so a joint some where, did not really want to use mains voltage, and wanted to control DHW and CH plus keep the thermostat/controller powered, and Nest Gen 3 has done that.
However after Nest went to Google it seemed to go down hill. My first problem was the anti legionnaires which would turn on boiler at random times, since there is no tank thermostat on my system it was clearly wasting energy, found how to turn off feature, then there was the learning bit, well doors open or doors closed changes how fast my hall heats or cools, so it built up a crazy sequence so that had to be turned off, and then the geofencing it seems it swaps from eco to comfort, but no way to set at how far away it swaps, it did seem to work of sorts, but early this year high winds took out the EE mast and it was second day before I realised why so cold, as when you walk past the thermostat it detects you anyway so goes to comfort, but sitting in my bedroom playing on PC it thought I was not home so went to Eco, so geofencing also turned off.
So all the smart features have been turned off. Since Nest is the only smart wall thermostat I have tried to use can't say how others work, but I am now a little wary of using smart features.
We also have Nest Mini's great for listening to radio, or other music, and use it a lot to help with spelling, but as to working smart stuff, it was like a mischievous little boy, phone rings so tell it to turn off, and instead of turning off music it turns off 5 switches which is the lights, hay google turn lights back on gets "don't know how to do that yet." so not keen on putting central heating into the control of Nest Mini's, I am sure both the TRV's and the Nest wall thermostat can both be controlled with Nest Mini's but I want a warm home.
So it seems likely you can use some thing like alexa or nest to integrate different makes of central heating controls, I know energenie uses IFTTT which is another way to integrate different systems, the big question is do you want to trust your heating to some mischievous little gadget which may result in a cold home? Specially when programmed with speech so you can't see what it intends to do.
I did look at Tado before fitting Nest, and it seemed they did not want to tell anyone how it worked, little better now, but after Nest played silly, not wanting to trust some thing which I know so little about will do what I want.
I thought with geofencing I could have set it to work in a step system, so up to 10 miles from home drops 2°C and 20 miles 3°C then 30 miles 4°C and so on so giving the system time to get back to comfortable temperature before I get home, but nothing so smart, so in the main it is take out phone and manually alter the heating/cooling level.
I watch my wife returning home on google maps to know when to start dinner or make coffee, but it jumps, specially last 8 mile, so see her in Welshpool, where she may stop and do some shopping, which is 8 mile away, and next thing it shows her in my town, I assume google maps uses same info as geofencing so can see how unlikely to work as expected.
