I'm intruiged by the concept of under carpet heating. Having recently undergone a side return I debated long and hard with our builders on the floor finish. We wanted underfloor heating but were advised that the timber flooring we were proposing having in the kitchen is a bad conductor of heat and as such much of the heat from the UFH would be lost. In the end we went for a timber look tile (limed oak) which is fabulous and allowed us to have UFH. The tiles heat up nice and toasty and provide good ambient heat into the room.
Given that carpet / underlay are even worse conductors of heat than wood I'm really surprised that a builder is proposing UFH under a carpet....unless there is a new system I've not heard of.
Re electricty bills there are 2 systems - a wet system which utilises hot water from your boiler and electric mats laid under the tiles / wood. The wet system is far more economical to run than the later but is more expensive to install.
Last point on UFH is that you must make sure your builder insulates under whichever system you use. This ensures that the heat produced doesn't heat the concrete / void under your floor but is reflected up to heat whatever floor covering you have......
Good luck anyway and keep us posted.