Is Polypipe the best brand?
And we’re laying on top of an existing floor so it goes screed, old tiles, wooden structure with insulation, underfloor heating, new tiles – is that the best in terms of insulation?

Probably not, have they done any calculations for you regarding heat loses, floor temperatures, pipework layout drawings?

There are heat loss calcs for the room itself (addition of large expanse of glass) though I dont think so for the floor itself.
Is that something to look into? I couldn’t see many other options than what they are doing tbh.