The underfloor pipes go under the screed above the insulation.

Yes they must be filled with water and under pressure when the screed is laid.

We worked on a site recently which was a small block of flats - only 4 story, 2 flats each floor.

The plumber was fitting UF heating with an electromax electric boiler (yup).

The plumber and builder laid the insulation slabs and UFH on top of the concrete slab / block and beam.  The pipes where secured with with the push in clips which are meant to grab their own fixing in the insulation.

It looked a right mess, and most people on site commented.

They had arranged for a company to screed the floors due to access.  The company where using a product I am not familar with which meant parking a truck outside and pumping a self levelling screed upto the flats.  Shuttering was installed where required, and the screed poured.  No troweling or leveling required.  Just pour and walk away.

There was all kinds of problems with floating pipes AND FLOATING INSULATION boards!

Never seen anything like it!  Men running around in wellies trying to push the pipes and board back under!  Using anything and everything to keep it down!  They had to do some work to repair areas afterwards where they weighted down the pipes/board with concrete blocks!

Not good!

The insulation manufacturers also recommend a plastic sheet on top of the insulation before fitting the pipes to stop water from the screed getting into/under the insulation.