Electric underfloor heating is rarely a viable due to small free space in a bathroom :-)
#1 - Insulate the walls to 40-60mm Marmox (there are cheaper clones). The transformation is simply enormous - because instead of trying to heat the walls & tiles you only need heat the air in the small room, which is must faster & much easier. Internal insulation has a *very* substantial effect.
#2 - Avoid tiling on the floor, consider a warmer wet room vinyl surface like Alto Marine (it is not expensive).
If heating by electric... Set the towel rail element to provide a low background temperature, which is boosted on demand. This can be done manually by a 600 Watt MEG element on a 1200x550 WHITE towel rail; you set it to 30-37.5-45oC for most of the time and only push the buttons to make it run 60oC for the periods when it is very cold.
If heating by water... Fit a double radiator or such like to boost performance.
HOWEVER, a bathroom conversion from "no insulation to 40mm Marmox" needs very little heating. As you put thick marmox up you suddenly... become aware... the cold is going... and you are getting piggin hot... even when you were shivering before with freezing brickwork in front of you. Just from your body heat. Quite surreal and catches you out when plastering or tiling because things... suddenly... begin to go off very very quickly if thermally activated :-)
The vinyl wet room systems are superior to tiling - you can get non- slip tiles, but vinyl feels warmer and done properly it is the solution of choice from hotels to NHS to disabled. Forget underfloor heating, insulate heavily + towel rail or proper radiator.