Wet Under floor heating

I am planning to fit single room ufh in my extension - problem is the 100mm celotex is already under 50mm concrete slab - if I screw down the pex pipe using copper 2 hole pipe clips directly onto the slab and lay 50mm screed on top will the ufh heat the room (knowing that I will have some downward heat loss until it hits the celotex) I’m aware of the overlay boards but trying to keep the cost down and the boards would be £400

You might be able to break up and dig up that 50mm concrete slab in a few minutes with the right tool!

Aldi do a breaker which looks the same as the draper one, and it's £80. Or hire one!

(may be more £££)

Nozzle

I'm no expert but.....I'm not sure I would regard that as heat 'lost'. Eventually it will find it's way into your room (assuming there is no thermal briding issues). The floor will just react slower to a change in temperature because it's heating more mass, but IMO it would be fine. Could always use closer pipe spacing to compensate.

A screed layed in two 50mm layers? I guess it would be fine, but worth looking into maybe.

But as Nozzle said, probably wouldn't be too difficult to get it up.