I'm not sure exactly how budgets for lounges are allocated, but in the current climate, I'd say the chances of a BA taking on a new lease at a short-haul outstation and incurring capex on fitting out its own lounge are quite low.

The trend over the years has been to use more partner airline/third party lounges than opening new lounges. In terms of "brand presence" it's better for BA to have its own lounges, but it's much easier to just pay a periodic service fee to a partner than go through all the hoops of negotiating a lease that complies with local legislation and incurring up front fit out costs etc.

Across the network, as well as premium traffic, I guess income from partners helps fund lounge fit out and running costs.

At YUL, the third party lounge BA uses is also a Priority Pass lounge.