The same thing happened to me today. This must be a new policy.
The agent was very apologetic but said only Gold can access the BA lounge in EDI when flying Qatar. Business and silver being sent to Turkish lounge which is pretty mediocre, especially for food.
BA Lounge looked dead as this was 5am in the morning fwiw, so not a capacity thing but a broader policy.
We were 2 silvers and 2 non-status passengers in J on QR. Didn’t have the energy to argue at that time in the morning. I suspect if I had just started scanning the bps myself they would have worked.