While I haven't been to the dentist yet, after living in UK for a year I can testify that the bar for cheating or misleading customers is very low.
As an example, taxi drivers usually pick a longer route than necessary, sometimes driving around the whole city. Especially if they perceive that you are not local.
I thought it might be that many practices are both NHS and private and you can have some things covered but get some private work done at the same time.
I can't comment on the taxis much but all will be regulated with the local council so that would be a point to start a complaint. My only note is that some cities like Liverpool or Manchester can have major congestion which needs avoiding, as well as (particularly on Liverpool) seemingly insane one way systems.
Uber and some other local versions send you a receipt with a map of the route taken which you could complain with.
Consumer protection laws are pretty strong here. I can dig out mode specifics if it'd help.
However I myself have not been ripped off by an NHS dentist - mainly because I am aware of the bands. I would certainly take it further if they tried to.
There are regulatory bodies specifically for dentistry as well as for the NHS and sales and advertising. I would heavily suggest that they report this, there seems to be places to do so and I consider this an extremely important issue to solve if it's happening.
I'm not saying you're wrong, just that your friends experiences are so vastly different from my friends.