My problem is that the current 'slab' height means that it will
be impossible to raise the floor by any more than 100mm.. So,
If I have to add 50mm of insulation and 75mm of screed, the only
physically way possible would be to lower the slab by 25mm, and
as you will imagine, this impossible.

Well, 25mm of insulation would be infinitely better than none. That would
allow 75mm of screed.

Christian.