Do you want quick response? Then dont use UFH -- if you are happy with 24/7 comfort then put the insulation under the concrete and ignore the plumber

Insulate a whole lot better than they tell you if you are going to stay there.

We've lived with our UFH system for three years. I'd go with your plumbers advice.

What floor coverings are you going for?

In areas with a wood floor covering you can do without the screed. Fix say 80mm deep battens to the concrete slab. Between these lay Osma foiled insulation system or similar system with heat spreader plates. Nail and glue the finished wood floor to the battens. Works well. Good response time and maximium thickness of insulation. Wood floor must be >18mm thick to avoid flexing. Avoid any unnecessary layers - don't go for a thin laminate that would need 18mm chipboard under it as well.

Carpets: We discovered that (contrary to what some people say) you do have to be careful choosing carpets. Several carpet salesmen said their carpet and underlay was suitable for UFH when it clearly wasn't. What they mean is your UFH won't damage the carpet not that the carpet will let the heat out. You need to source low TOG underlay (0.6-0.7) and keep the total TOG as low as possible. Ideally below 2 but no more than 2.5. Ask to see the makers data. Most places don't have the TOG info info to hand and will have to look it up. Ideally check it yourself with a call to the manufacturer. One sales rep we spoke to obviously had never heard of TOG but rather than loose a sale he made up the info. Told us it was twenty seven or some such nonsense.

Stone and tile: Best choice with UFH. Works really well on screed. Watch out if tiling through a door way between two adjacent rooms. We had a problem with the screed in both rooms shrinking away from each other. It cracked at the door way and the crack propagated through the stone floor. If doing this again I would stop them screeding through the door way and insert an expansion gap. Then arrange for a grout line in the tiles/stone to be over the gap. This would encourage any movement to occur at a grout line. Our builder covered the screed with plastic sheet when it was laid to carefully control the drying process and we allowed several months for it to dry out properly so that wasn't the cause.

Bathroom on timber first floor. Make sure the builder knows if you are going to tile it. Tiles can only be laid on a rigid wood floor. To make it rigid extra noggins and one or more layers of marine plywood are normally screwed down. Not great for UFH but should work. I wish someone made better solution. Perhaps a structural cement board that conducts heat better than marine plywood? We oped for a beam and block first floor as well so it wasn't an issue for us. Heatmiser make an UFH stat with a remote sensor. Allows the stat to be outside the bathroom out of all the steam.