As a PPL(H) businessman (but one who takes additional training sorties every year, the most recent being this month due to no stick time in December).
I think for the UK Bell Ringer makes a point that resonates for me. In the UK we are rarely hot or high (yes, it is possible but rare). I can see that it is possible to both fail to appreciate the practical application of what you are taught about their impact on performance, or "forget" as you rarely apply the theory. Also, flying mainly solo or two-up in a machine with significant excess power / performance for that weight can also lead to sloppy standards because you don't get punished for your errors. Put those together, and take a trip at MAUW without adequate planning and you can see where that trip will end ........ far short of the planned destination.
I am fairly sure that I don't agree with the "standards of instructor in the UK" complaint; but that is based on my personal experience.