Honestly, I would question if whatever smart systems you are implementing are worth the money. £400+ for two Nest thermostats - how quickly will they make that money back vs a simple timer+dumb thermostat equivalent?
It's more down to a change in heating philosophy, I replaced a dumb timer and 2 thermostats in my 2 zone house in Jan this year with 2 nests, I have already saved over £100 total in monthly reductions on my Gas bill.
The heating is only on when we are in, for a young family this is a God send with erratic schedules, when we are not in the thermostats drop into eco mode only heating the property if the temp drops below 12c. Before it would come on if we were there or not.
The digital thermostats are far more accurate than the mechanical ones, the mechanical ones we did have had about a 4c accuracy, the nests have a 0.5c accuracy, some maintaining the temperature you want is far easier.
Additionally with nests it learned a schedule and calculates heat up time of the zones based on learned data and external weather forecast, this ensures that no one is pressing the boost buttons or artificial increasing the setting to get the zones to temperature quickly.
And one last thing, the app on my phone easily allows me to see if one of the kids has played and I can remotely correct the setting, before they had a habit of fiddling an we would not be aware until it was too hot.
