To my mind Hive and Nest are really for hot air systems as used in USA, for the British system you need to control boiler flame height, there are two main ways to control boiler output, OpenTherm will allow a wall thermostat to control boiler, but most boilers are not designed for OpenTherm the other method which is used by most boilers is the return water temperature.
To control return water temperature we use thermostatic radiator valves (TRV) you can replace the head on the valve for an electronic version, the electronic version measures air temperature and water temperature so can compensate the air temperature measured to allow for heat from radiator so they are actually set to a temperature not some random number. Also you can set a program so temperature varies through the day, i.e. no heat in bedrooms during day and no heat in living rooms at night.
After this we have some stand alone under £30 each, some are WiFi around £60 plus a hub, and the bees knees connect to a special hub called a thermostat which takes the demand from each TRV and decides if the boiler should run and if using OpenTherm at what output, that is EvoHome and although very good also very expensive.
Hive and Nest do have a work around it is called IFTTT where they can using an internet server talk to TRV heads like the Energenie but with your internet connection that is not really a good idea. All you want is a remote switch, heating on or heating off with maybe a thermostat bypassing switch as frost protection.
When I look at the hub for my Energenie the light is flashing all the time, as to how much it uses internet I don't know, I did a google for "sim card controlled switch" and found devices at under £14 that sounds a better idea to me, you can switch on/off but have all central heating controls local.
The other option is a timer so one push and it works for 24 or 48 hours and when in the holiday cottage you push it once a day.
There has been a thread where IFTTT was talked about, it seems there is a follow command where the Nest follows the TRV or the TRV can follow the Nest which means unlike EvoHome every room is at same temperature but it does mean change the temperature using your phone on one device and all the others follow. However there is a problem with most broad band in that the IP is not static, so to get around that problem a remote server is used, so each device asks the remote server what should I do, so home or phone talk to this remote server. This means a lot of data is passed, and for heating to fail because the sim card has run out of it's data allowance is not really what you want.
