You were very fortunate with BA. They're concerned (too concerned, IMHO, but they didn't ask me) that someone may have dropped a level between booking (which boarding passes normally reflect) and when they want to use a lounge, so they generally insist on a real, current card with no substitutions allowed. When that happens, logic will not sway them: they care about their job more than they care about any one passenger, especially one who usually flies with another airline. I haven't personally had to deal with that problem but it's been reported here so often, at so many different airports, that you shouldn't be surprised if it happens.