far more complicated than they are making out
First we have sync speed, this is affected by two things, line quality and line length. Now line quality is down to openreach so it seem fair enough that if your quality isn't right you shouldn't have to pay. Line length is down to the physical locations of the property and the exchange whose fault is that? If the line is correctly sold, i.e. you are told at the time what speed to expect then I don't see a problem.
Next you have contention, oversubscribed exchanges, backhaul peering, how do you test for that on an individual basis.
Then you have slow websites, my Internet in my house runs happily at around 12Mbps but I don't see that all the time and it's not down to my ISP.
However there is something else that is far less complicated to solve and that's use of the word unlimited. The best way to solve that is the same way that the mobile providers have with broadband dongles by selling a fixed amount of data.