How to overcome 7-8mm height difference

We have existing flooring which will be about 7 to 8 mm higher than new tiling (once tiling is installed).
What is the best way to avoid having this 7-8mm height difference? We spoke to a few tilers and they say that the glue itself cannot make up this much of the height, "no way" they said. In fact, they said that the glue layer will end up being negligble, once the tiles push the glue around. So, we can almost forget about the thickness of the glue.

I heard of "ceramic tile underlays". But, we will be using porcelain tiles - so "ceramic tile underlay" can't be used??

Also don't really want to use the self levelling because it apparently needs etching of the slab and what not, just sounds too risky and with all the other things in place already, the potential to some accidents is quite high.

Finally, I've been told that the minimun thinckness of concrete screed should be 25mm (otherwise, it will crack and that would be felt as drumminess under the tiles). So it looks like we can't use that either.

What do you suggest to even the levels with? Or do we just leave the tiling lower - in which case we have a small problem with the skirting sitting on 2 different levels!

Finally, the slab is not that level to start with

Thanks for your comments!