Genghis, you said "...differences training doesn't require the instructor to train somebody to pass a skill test again..." but it does require the training to "a level of competence appropriate to the holder of an NPPL M", which amounts to the same thing - in other words the instructor must train to the level at which they are confident that if the pilot were to be assessed by skills test then they would pass it.
Having said that I reckon that about 25% of pilots that I take for their bienniel training hour couldn't pass the skills test if they took it again. But that's another can of worms entirely.
FWIW I agree that the route to licences starting with the microlight NPPL M is a good one. I know plenty of folk who have gone that way and surprised themselves by finding they didn't need or want to go any further when it came to it - because they could do all they wanted to do with a microlight.