And that means that introducing per-appointment charges would likely have only a very minor effect on reducing waste & improving availability. Healthy people of working age would see it as imposing extra charges for no tangible improvement.
Either that, or you'd need to be looking at introducing a two-tier NHS, which I don't think many people would be happy with - "it might work in Europe, but what if it ends up being more like America?" would be the worry.
I think your point about the importance of triage to being able to running "lean" is a better place to look for improvements. The Single Patient Record ought to make automated triage a lot more reliable, for instance. And taking a broader view of ongoing health & economic impacts would help with your ADHD/autism assessment use case, as well as similar issues affecting people of working age (maternity care, being another obvious example).