towel warmers give out much less heat than radiators. Even less when they are lagged with a thick insulating layer of towels.
If the room is already being heated with a radiator or some other means, just get as big a rail as will fit. Wider ones are better than high ones as the towels do not need to hang over each other.
1kWh is about 3400 BTU
I work in Watts, not BTUs. Radiator salesmen like to talk in BTUs because the numbers are bigger and look more impressive. My car weights about 35,000 ounces.
300W is about enough to heat a WC cubicle. Digital heaters and wifi control are useless gimmicks and put up the purchase price.
A 1kW heater should be adequate for an average bathroom, it needs a thermostatic control because a lot of the time demand will be less.
If you put in a radiator (preferably a wide, low one) you can put towel rails or rings on the wall above it. Rising heat will warm them.
A 24kW boiler will have the capacity to give out more heat than any ordinary modern house needs, so don't skimp on your radiators. Large ones will heat your house more quickly from cold.