I would think that the UFH supplier would install pipework, manifolds, thermostats and controls but your builder would install insulation first and then the screed. There are many UFH suppliers out there. Send your drawings and they will give you a quote. Do a litte research first and look at your requirements re zoning, floor covering, controls, thermostats etc. 100mm of kingspan or similar seems preferable haing read other posts. There are different screeds available also and you could have your architect/engineer advise. I would insulate the screed from the external walls also by placing insulation vertically around the perimeter of each ext. wall.