This happens at outstation lounges. It's a particular problem because BA hasn't suffered the same level of precious metal inflation as most other airlines (Gold on AA doesn't get you into any lounges at all). Slightly surprised at Porto as it's normally a bit better at places that also have BA service but I had the same at Riga yesterday (although the lounge receptionist was very polite and I eventually dug up the Finnair invite on the in-app boarding pass).