Underfloor Heating (Concrete floor). What's the correct layers to apply?

The above would work fine however the self levelling screed would not be necessary. Most heat mats state that the tiles can be laid directly onto the heat mats using a flexible tile adhesive. But it’s very important to ensure the mats are completely encapsulated by the adhesive with no air pockets. The advantages to covering the mats with self levelling screed is that you can be confident that the mats are completely encapsulated. But do be aware that self levelling screed can be problematic if not properly primed before laying the tiles

As the cost of digging up an old floor is costly, yes the route of adding insulation boards below the electric matting is the cheapest alternative. It does increase the performance and also saves on those electric bills a little bit as more heat is sent up and not lost through the ground. If we are preparing for tiling, we typically would go back to the original screed finish, then adhesive the insulation boards to the screed, using a 10mm notched trowel serrate both the floor and the back of the insulation board and bed down. You must use the right type of insulation board for the application, there are different types for wood floors for instance... Using a flexible primer is important, normally used on the original screed before you adhesive down the boards, but also on the insulation boards to prime them for the SLC Then apply the underfloor heating mat directly on top of the insulation, then pour over a good quality flexible self levelling screed. Then it’s ready for tiling, always use a high flexibility tile adhesive too as heating and cooling makes things expand and contract, and don’t have the heating mat on when you’re tiling... let the adhesive cure maybe a week, before turning on the underfloor heating and turn it up slowly bit by bit. Some underfloor heating controls have a first use setting to help with this.

prime the floor /good self leveling screed suitable for underfloor heating and make sure the liquid screed cover all the wires

I agree with 90 per cent of the answer but it is very important that all adhesive and grout is flexible and that a sbr sealer used

As iv installed underfloor heating for a conservatory company the policey was to fix insulation boards with flexible adhesive ensuring the heating mat would stick then flexible adhesive and grout