The problem with using average as you suggest is that you have to assume it’s a constant across all routes/times/directions (the presence and ‘random’ use of A319/20/21 suggests it isn’t). The bigger assumption is that the average fare across the entire SH network is equal. BA, like most European airlines has another variable, they can vary the size of Business Class. I don’t know if BA do, but my impression is that LX make the Y cabin as small as possible to accommodate the booked load, often leading to large, sparsely populated C cabins. This makes some sense as if you’re going to get a free seat next to you, you might be less inclined to book CE.

I guess the short version of a long story is that BA already offer extra legroom in Y for those who want to pay, or more space in CE. That golds booking a few days out can still get exit rows and CE is usually smaller than ET suggest most people are prepared to sacrifice comfort for cash.