The question of insulation depth is a bit academic since Building Control will have to approve the overall insulation system for the whole house, of hich the flooring is but one part.
The sequence from the bottom upwards is slab, insulation, waterproof membrane, screed, heating mat, final finishing (tiles or whatever).
You use the words 'heating pipes' which is confusing - no pipes in an electric system. If you are using water-pipes, these go down on top of the insulation and are then embedded in the screed - either about 65mm normal screed or 40mm anhydrite screed.
The minimum insulation thickness will be 70mm, but more is better - you have to remember that a warm floor will transmit greater heat downdowns into the subsoil than a conventional floor.
Electric UFH is not a cheap system to run - it's the basic ecomonics of using expensive electricity.