I've never heard of the term Black Friday in this context. When I used to work in Whitehall some years ago the busiest day would have been next Friday (i.e. the last working day before Christmas), but I suppose that it has now been brought forward with the Christmas break now being two to three weeks and workers finishing so much earlier. I always thought some pubs actually lost money by being so very crowded. I've been in pubs when you couldn't see the bar counter let alone get to it. But if you were lucky to get a spot at the bar you stayed there, and people at the back were lucky to get one round in. Thus you had lots of people in the pub not drinking and unable to get served, and just getting in the way of those more unfortunate souls even further away from the bar counter.