IMHO. Maybe it's a flaw with my generation or maybe I wear rose tinted glasses, but most people my age do not seem to play the long game and are not prepared to invest longterm into their career and await the fruits of their labour.
The problem with this argument is pretty obvious really. I became a captain when I was 30. Running my take home pay backwards through a UK salary calculator shows that I would have to earn well over £200k a year to have the same net pay in the UK, and I work part time. Granted, I live in an expensive part of the world, but not appreciably more so than London.
I'm sure there are BA captains who earn more than me, but they'll have been in the company a long time, and almost certainly have training and/or management responsibilities (or do a hell of a lot of overtime!). The power of a (relatively) high salary when you're (relatively) young is in compounding (the same reason why sticking money in a pension is a good idea at the earliest opportunity). Financially it simply wouldn't make sense for someone like me to make the move; yes, I'd probably earn more for the last few years of my career, but I've got around two decades a of earning more. All assuming Ts & Cs remain static of course!
As others have eluded to, going to BA certainly wouldn't be a decision based on finances for many. This is all to say nothing of the other reasons to make the switch.