Underfloor Heating cannot be retrofitted into an existing bathroom. However, if you are renovating or changing the floor finish in your bathroom then an underfloor heating system is certainly something that can be added.
One of the advantages of Thermonet undertile heating is that the cable is only 3.5mm thick, making it ideal for bathroom renovations. Even, if you are not removing your old tiles, we have helped many customers over the years who have installed Thermonet on top of their existing tiles before re-tiling. Most of the 3.5mm cable thickness will be absorbed in the tile adhesive bed meaning the final floor height will only be about 2mm more than without floor heating,
It is also important to ensure that there is enough power available on the existing circuit or that an additional power supply circuit can be run to the bathroom to allow for the floor heating.
If you have completed your bathroom and you did not include floor heating and now your bathroom is cold you could consider installing a fan heater. These can be easily retrofitted on the wall in your bathrooms and while your tiles will still be cool at least your room will be warm. See details on fan heaters here.