I don't need a lawyer to tell me that I am being reasonable and BT charging excess after they have stopped providing service is being unreasonable.
I don't think anyone here is suggesting that the situation hasn't gone wrong, nor that (somehow) you have been charged more than you should have been (had it gone correctly).
Surely the question is how do you get your money back when virtually all of the "evidence" has been via Telephone Calls (rather than anything specific in writing)?
As I'm sure you will appreciate that, unless you have specific recordings of the conversations, these can always be open to interpretation.
There is always the danger that someone has "assumed" something, whether that assumption was totally intended or not!
Clearly I was not a party to your conversations &, hence, I will not pretend to know whether you gave (or did not give) the right information/instructions at the requisite time.
My only suggestion would be to contact BT & see what you can achieve. Be polite but firm - DON'T try to get "All-Contractual" & DON'T try to quote OFCOM as being "Tablets-of-Stone" criteria (they are not - they are "Guidelines").
I would also suggest on trying to get the bulk of your money back & NOT get too hung-up & trying to achieve every last penny (surely it must be better to get 95% of the monies, rather than force them into a corner & forcing them to object to everything?