The problem is so many variables, assuming it was fitted correctly i.e. some one band jacked the floor up and fitted insulation first before fitting the underfloor heating mat, then what goes in must come out into the house, so running cost is simply difference between gas and electric price assuming gas central heating and fitted to ground floor.
But since heating up times are so long, you would need to run for at least 2 hours any shorter is waste of time, and this leads to how different forms of electric heating is compared. Forgetting heat pumps and inferred all electric heater give out what is put in, so run 24/7 they all work out the same. Where this changes is where not run 24/7, so a room we will say is inhabitable when the temperature is 18°C even if idea temperature is 20°C, we will assume it will cool to 14°C when heating turned off for 12 hours, so difference between heating methods is speed, room used for 1 hour and it takes 1 hour to heat then 50% efficient, if it takes 2 hours to heat then 33% efficient. But used for 2 hours and one hour to heat then 66% efficient, so because underfloor heating takes so long to heat the room and a bathroom is used for such a short time the efficiency is very poor with underfloor heating and best with a fan heater.
However in my bathroom the underfloor heating is not really there to heat the room, it is there to dry the floor, so with underfloor heating 2 hours after a shower the floor is dry, with a wall mounted fan heater it would also take 2 hours, but fan heater is 2 kW and underfloor heating is 800W so underfloor heating is cheaper, however open the wet room door and switch on the extractor fan and dry in 4 hours so that seems the cheapest option until you start to count up how much gas is used to heat the air blown out of the vent.
Cheapest method is a mop.
As far as warm toes go, well as soon as the shower is turned on then the tiles are temperature of shower water, it really does not matter if UFH is on or off.
As to running cost, there are two sensors in the thermostat, one buried in the floor to ensure the floor does not get too hot to stand on, and one in the room measuring air temperature, at which point each starts to cycle I don't know, to measure I would need to disconnect the supply in the consumer unit and use a plug and socket so I could fit a energy measuring device, or use a clamp on measuring device.
To quote "If on constantly - £21.50 per week" which is unlikely, so the question is, does it use enough to worry about? A simple electric clock connected to thermostat output would show how long the mat is powered for so from that you could work out what it uses, however to connect the clock in a safe manor and go to all the trouble to measure for a max bill of £21.50 per week seems to be going a bit OTT. If I want to watch TV I simply turn it on, I don't work out how much it will cost, same with UFH do you work out how much it costs to dry your wife's hair? It could likely cost the same as using the UFH while having a shower, I would never recommend fitting UFH but if already fitted then why not use it?
