Contact the company who supplied/manufactured the underfloor system to find out if they do a room temperature sensor that can be wired in to the system. If not a room thermostat could be fitted to interupt the electrical supply to the matt. DO NOT FIT A STANDARD ROOM THERMOSTAT CONSULT AN ELECTRICIAN TO CALCULATE THE SWITCH RATE (IN AMPS) REQUIRED TO SWITCH THE POWER OFF AND ON. The current you are switching may be a inductive current & not a resistive current which means the contact rating of the thermostat will have to be increased to suit. You will also have to fit the type of thermostat that has a remote sensor to it as you can not put a standard room thermostat in a bathroom.