Hi,
I can empathize.. but I also do think that Gracie is right.. BA does speak to this issue in their Contract of Carriage, Section 8j:
BA CoC;
britishairways.com/travel/…en_us
The notion that you have used it before without issuse is unfortunately not germane.. but.. I do also cede that the inconsistent enforcement of carry-on baggage makes this issue even more difficult to police.. but in the end, your prior flights with this bag does then make it acceptable for all future flights-- each one is it's own unique situation.
As far as the staff not being helpful goes.. that's hard to say.. I will say that if this happened well before check-in cut off time, perhaps more could have been done to help you find suitable packing materials.. but we'd need to know the time-frame in which this occurred and the situation at that time.
The size of the template itself may or may not be "ridiculously tight" but in the end, unless there is a local rule to the contrary, BA is free to set that as small as they see fit..
While I'm not saying that perhaps BA could have done more for you at the check-in counters to try to find alternatives that still met their rules, but also better protected your pottery, in the end, knowing and complying with carry on baggage rules do fall to you the passenger.. While BA doesn't apparently publish one set size limit, they do make it known that there IS a size limit..
Travel Safe,