1950s concrete slab prep for insulation

[NandM]

I've removed the screed from the ground floor and have a relatively good slab...except for the transition into one of the rooms. By transition, I mean there are a course of bricks separating one room from the other

 

There seems to be height mismatch of upto 1.5cm at some points and I'm unsure what to do about it.  Couple some self leveling screed or a light cement mixture be used to help ease this?

 

I could start/stop the insulation boards at the transition point and then maybe use some 12mm OSB to level the floor. I plan to have a floating wood floor finish.