It depends very much on which bank you're working for. American/Asian banks tend to be much more hard pushing in terms of hours of facetime you have to put in, but at many of the European banks 40 hour weeks are fairly average even for Front Office devs.

Cost of living wise london isn't that expensive once you take into account cost of commuting. I'm paying £1300/month for a large two bedroom flat within walking distance of the city.