Re: Why only one thermostat?
re the security comment at the end..
"yes, there are possible security issues, but stick the system on a different vlan and life is made that little trickier for a potential attack..."
EvoHome can work completely off grid depending on your flavour of foil hat.
There is a colour screen controller unit that can talk to the Honeywell SaS mobile interface thing, or just use it standalone with the colour controller thing and the room thermostats.
AIUI, the comms between the thermostats and Rad controllers (TRV's) is not WiFi.