£140,000 a year for working 7am-midnight (or whatever the crazy hours bankers work), including lots of weekend work, doesn't really seem worth it in the grand scheme of things...
Sure, £140k is a lot more than most people are getting, but it's by no means a huge amount of money... especially with the costs of living in London and the SE being astronomically high.
People on £30k may earn a minuscule amount in comparison to bankers, but these people tend to have better work-life balance, as well as leading healthier (both mentally and physically) lifestyles in general.
Whether you earn £30k or £100k pa, you're making money for the company you're working for first and foremost, and you're always going to be a little bit screwed over by these companies. It is the successful entrepreneurs and the CEOs that make the millions, not the people working for them...