Nu heat Lo pro max. Has the best kW per m2 for a 25mm total build up.
In this case, though. I would recommend an odd apraoch!!
Firstly I would bin the timber joists and fit a block and beam floor.
Set the level of this floor 160mm down from finished floor level . Fit 100mm insulation as per building regs.
Then for the existing floor ( bear with me) I would fit 25mm insulation ( I know it's not perfect, but it's the best that can be done). This will line up with the new extension floor. Cover with membrane and fit standard wet underfloor.
55mm screed on top.
Somtimes bending the rules works. Not perfect, but works.