right.. an ASHP... I thought that might be the case.
It will all depend on what insulation they used - whether the UFH is wet or dry system... but you might find this link helpful as to how an ASHP works...
http://www.itwontcost.../home-cleantech/ashp/
Your costs for electric will be to run the pump itself.. the electric doesnt do the heating bit.. the Air does this! SO yes it will be cheaper than gas or electric... since its using the air temperature to heat the water etc.
No-one for sure can say what it will actually cost - but ask to speak to the company who fitted it for some typical costs based on a dwelling of your size.