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My experience:
We've had electric underfloor fitted to our loft bathroom as a 'we'll see if it works and how much it costs to run' sort of experiment because: it wasn't expensive to fit; and we have the back-up of the (hot water) towel rail.Over the winter we found the underfloor to be very effective to take the edge off in the mornings and evenings and it hasn't added greatly to our 'leccy bill. Can't comment on its use as the primary heating source though.
Yay. But, I'd think you'd want a heated towel rail in there too (or instead, I suppose, if its a big enough one).