"as i've written before, BT currently has an interest in keeping OpenReaches (Line Rental) charges down, ISP's will have an interest in upping openreaches charges so they get more margin on the passed through costs, possibly by sticking in unnecessary gear and charging their peers for transiting it."
No - BT have been inflating Openreach charges wherever possible (since that allows them to milk their "competitors") including inventing truly absurd charges, like the three or occasionally even four figure charge for reporting complex faults, "SFI". (By shifting goalposts, they now impose a charge if the first engineer fails to find a fault, even when the second engineer finds and fixes it then admits the first guy should have if he'd done the job properly.)
As for unreasonable charges, get back to me when I can dial a UK number starting 07 from anything other than another 07 number without getting gouged to bankruptcy for it! (Yes, to be fair, BT and Three are both campaigning against that particular scam now, but it's been in place a very long time now.)
It was only last year I had an "Openreach" engineer in my home telling me my fault would be fixed more quickly if I used BT as my ISP rather than one of their rivals!