We have it in our bathrooms too, we just have it set for an hour in the morning and evening, it takes an hour to heat the floor fully so it's toasty for showers in the morning and evening. It stays warm for a good while after it goes off and is not noticeable on the bill at all. Our electrician (bro-in-law) advised us that they were heavy enough on esb to leave on all the time.
Heated towel rails are the absolute bomb though imo, so def go for those -we got ours in a b & q sale and they were about 80 each for med-large ones. Snug warm towels and they stay warm much longer than the rads.
HTH