Hi guys new on here so sorry if someone has already asked this question before! So here goes....
I am building a conservatory a 11m2 well insulated concrete floor. I need to build the floor up for tiling and I'm fitting electric 200watt ufh. I have about 50mm to build and want it done properly. I have looked at tile backer boards but they are very expensive and thin boards. Also looked at celortex with 18mm ply screwed on top then primed and tiled would this be ok? I have about £200 to play with if it poss if not I will wait and save a little longer ha. I haven't got a clue which is the best way so an advise would be great. thanks guys.

Insulation boards would be the way to go. Marmox etc

ok thanks how do u fix them down can I tile straight on top of them?

What is the substrate now ?

Rough concrete

Lay marmox with single part flexible adhesive then lay UFH then cover ufh with flexible SLC. Then you can tile.

As above. Providing you follow the manufacturers guidelines, that will give you a much better substrate to tile onto then using ply.

As above for me for the easiest route.... click the link in my signature (PHG) for best price on 40 to 50 mm Marmox..

or screed it

Is it a well tamped concrete with plenty of high point.....as above screed.

It is well tamped concrete and rough I like the idea of marmox can I self level the floor 1st to make it better for sticking the boards?

Yes you can but it might be worth looking at how much it adds up to with the boards/SLC/adhesive. Might not be the cheapest way of doing it.

Ok what would the cheapest way to do it?

Screed will be the cheapest route but you have drying times and need the ability to screed flat... you could also wet fix but again needs skill to this method.

As above. Screeding would be the cheapest route but the boards would be the easiest.

The boards are very ££ and screeding will take alot of time up are they any other ways of doing it celortex and ply screwed on top?

Any way to do this will cost, It seems you want to do it your way, so knock yourself out. The guys here know what is right and what could cause problems at a later date. Y
You are the one doing the job, so its your choice.