I don't have a wiring diagram as it can be used for so many different applications.
The two smaller terminals are the coil, you apply or remove 230v across these to switch the relay.
The COM, NO and NC are the switch for the appliance. Usually Live to Common and NO or NC to the live input of the appliance (depending on how you want it switched)
No, as electric underfloor heating thermostats control two temperatures. The air temperature and the floor temperature. The Nest would only be able to control the air temperature. This would mean the floor would get too hot. Risking damage to the floor (as well as making it too hot to stand on)