It's quite difficult in my opinion (been a builder over 15 years).
They probably wouldn't know it's thermalite blocks from a visual inspection but it wouldn't really be in the spirit of the law.
You might be able to get a decent fixing if you glue threaded rod in to the blocks. But you've still got a gap behind the boards. They would be to be cut back to the block imho.
This stuff is okay for the price and you only need a normal caulk gun.
The other thing I would say is that if you use something like stixall or CT1 in addition to the rods/bolts to actually glue the cabinet to the block with you would really struggle to get it off but the feo might not accept it.
The problem with thermalite is that even if you get a decent fixing in reality a crow bar will actually damage the block if you try to lever anything. I guess it has to last 5 minutes.
