Now, my memory in advancing years is not as good as it was, and I've had enough meals in the meantime to make me forget what things were like pre-BS, but I'm equally not entirely convinced that the situation is appreciably worse than in the days of Compass. I'll temper that by saying that the vast majority of my visits to the LHR lounges are at breakfast time, and as others have said it's actually pretty difficult to severely cost-cut that, given how cheap it is in the first place.
That's somewhat counter-intuitive to a saving of £7m though, so clearly something will have given. What we don't know is what - iirc BS also do the staff canteens and might have cut more quality there, they might have greater purchasing power than Compass, they might be prepared to take a much smaller profit. There's simply too many possibilities to merely attribute the saving to a decline in F lounge food.
What damns BA here, in my view, is how much others have upped their game. Whilst cws might well grow tired of the LH offering in T2, a lot of BA/oneworld customers will be drawing their comparisons with CX in T3 - where the food, and the service staff, are an absolute country mile ahead of BA imho. Maybe there's also a domino effect from associated gripes - the clear degradation in alcohol quality and the condition of the lounges hardly look good against, say, the UA offering in T2.