thanks to all for coming back. Just to try to take it a bit further....
Consider the situation where the tiles are first laid on the wall and the adhesive is still 'wet'. If you remove the spacers too early the tiles slide down under gravity to close the gaps but they don't fall off. And the wall doesn't fall down.
So assuming the wall is vertical and tiling is started at the bottom of the wall (i.e. tiles supported on the floor), I'm thinking that most of the weight ( the effect of gravity) will be transferred down through the tiles. The adhesion of the skim is then necessary just to keep the tiles vertical.....i.e. provide side support..... rather than supporting all their weight.
I can see poor quality skim, and poor bedding (& vertical alignment) of the tiles would be important, the heavier the tiles......is this maybe what British gypsum are guarding against?
The current rules are very restrictive, ruling out the usage of most modern larger format tiles, and requiring replacement of the skimed wall?
So what would you do faced with skimed kitchen walls and task of installing 300*600 porcelain tiles?