As a surveyor I am professionally bound to give you a serious answer. It is however correct that insulating the underside of the slab will give you little benefit. Potentially you could foam inject the voids between the bovedilla blocks, but the upper solid part of the floor including the tile itself will still act as a heat sink and draw heat from the room and your feet. If you were installing the slab you could use 'Arlita' aggregate over the beams and blocks, but even this method will give a cold feel in winter. The best permanent solution would be carpet or even wood laminate flooring over a foam or special 'Actis' reflective underlay.

Infra-red heating systems can help to warm floors but may prove costly in electricity. A decent woodburner will help to warm up any property and you could opt for a model that includes ducted transfer of the hot air, which in turn could be pumped via pipes running through the floor block voids, this will however also be expensive and ideally a flexible grout would be used under tiles to resist cracking from thermal movement. I am afraid decent large rugs are the best temporary solution.

Now where did I put my slippers?..

Under floor electric heating costs 1 cent (100 W) per square metre per hour. For an average sized flat (100 metres) that's 1€ per hour. There are 1440 hours in a billing period so if you ran is 24/7 you are looking at 1440€.

My tiled floors used to be freezing in the winter but with the wood stove burning they are fine. They absorb the radiant heat and give it up slowly long after the fire has stopped burning. Even the next morning they are still toasty.