BA vs CX depends on what your plans are.
If you don't hold right of residency/right to work in the EU, forget BA.
The major advantage with BA as I see it, and has already been pointed out, is that you're straight in as FO on the 737/A320/321/319/318. So you log valid, recognisable hours from Day 1. That's a massive difference.
In terms of living costs, London is very expensive (no surprise there), but unlike HK, you at least have the possibility of living outside London and paying a LOT less in rent. Supermarkets very cheap, as are utilities and services (if you shop around).
Eventually with seniority, you can even commute from outside the UK (Staff Travel very good, if there are seats or jumpseats available anywhere in the cabin, you'll get them. Some cabin staff will upgrade you as you board..)
No brainer if you live in Europe...