If you'll forgive me a little rant...
I knew the lack of explicit T&Cs would come back to bite me and now it has.

For reasons beyond my control I am forced to push my trip back by two weeks. "No problem," I say to myself, "its an MFU and those are fully flexible" but no - ba.com wants to assess a $275 change fee for the privilege! My best guess is this is for changing the leg I did not MFU. Of course it does not actually say this anywhere. Furthermore I am very sure it did not say this when it did briefly show me the T&Cs during the original booking process. It explicitly stated
the fare is changeable/refundable for no additional charge. It made
no reference to that only applying to the one leg and it
did not state any additional T&Cs that could have applied to the other leg.
I would love to press this with customer service, but as I have no copy of the T&Cs I was shown during booking - none are available through MMB and none came by email save for a weak statement that "restrictions may apply" - I have nothing whatsoever to back up my position.

There is absolutely no excuse for not providing your customers with a copy of a contract you are holding them to. At best its incompetent, at worst unethical, and I suspect in many jurisdictions, renders it unenforceable too.
Furthermore, when presented during the process with the T&Cs for the new flights they are significantly more restrictive than the original. "CANCELLATIONS ANY TIME TICKET IS NON-REFUNDABLE" (their caps, not mine)

How can you possibly justify completely changing the fare rules, simply by doing the equivalent of crossing out the date and writing in a new one? As far as I know they aren't even allowed to sell utterly non-refundable tickets in this country - even the most discounted buckets have always been refundable less a $275 fee in WT+ or $500 in CW.
One final (rhetorical) question - why is it that the fee is listed as $275 but it only wants to bill me $271.88? The only explanation I can think of is the US$/C$ exchange rate. But why am I being charged in US$ on a fare bought in Canada, in Canadian $ by a British company? What does the US have to do with anything?
I feel a little better now... still can't actually bring myself to click the button though.