Unofficially it has been done - take a look at [RELEASE] - Hive (Connect) V2.6 (British Gas Hive) which is a user-created device type and also a Thermostat SmartApp.
I have to say it works really well, you can do both heating and hot water, there’s a boost facility, interfaces with any other apps that are aware of a thermostat - for example I control mine through SmartTiles.