I get where you coming from, once a line is capped, then your point is that they no longer running the line "as fast as it can go", as they restricting it, I think you do have a point, but the problem is how these products are sold and how the system is designed to absolve responsibility.
e.g. openreach have managed to have no accountability to the end user at all, which is absolutely amazing for them they have a lot to thank ofcom for. On the flipside, the CPs have the ability to say they are powerless to do anything as they dont own the infrastructure, and even have a NDA pushed to them from openreach giving them a great reason to not tell their customers details.
Ofcom seem only concerned about two things. Low pricing and retail level competition, they believe increasing competition is enough to drive low prices, so they dont usually enforce prices at the retail level but just make a system that encourages people to change providers regurly and to drive down costs.
This has led to what we have now, a service on best efforts, sold as cheap as possible, with low levels of support from openreach.
Your isp will probably just throw the estimated speed at you and remind you that isnt even guaranteed and leave it at that, ideally for them you wouldnt even know if you banded in the first place.
Would I be bothered if I was banded at 35mbit on the 40mbit product? probably not, I used to be banded at 74mbit and I didnt do anything about it.
The situation some plusnet customers had as well where they ordered up to 76mbit and then got provisioned on the 40mbit product because they had low estimates. I dont think anyone managed to get compensation or out of their contract for it, but some managed to get reprovisioned on the proper package.