It amuses me how people on here comment on something they have no experience of...
Having lived in London for nearly 10 years, I am glad I have experienced it for its highs and lows.
We've just rented out our property in Greenwich and although it is expensive in the South East, it isn't as expensive as other areas e.g. south west, north etc
It depends on your situation and like others have said - your goals. If you're young, free and single - London is a great place. If you're looking to settle down and have kids, then maybe not so great. Although, having said that, people have and have survived. There is so much to do in London, but part of our move is that we had had enough of London and didn't want to raise kids in the smoke.
If you're going to be based in St Paul's then Greenwich/Blackheath although costly is a good shout... there are a few good places on Central Line too - Bethnal Green maybe? Central Line or any train to Cannon Street and St Paul's is a short walk - you could even cycle to work! Also, anywhere that has access to the Thameslink (is it still called that?!) calls at Blackfriars which also isn't far.
I really liked Spinningfields and see where you are making the comparison. The Oast House hanging kebabs are great!
One thing I agree on that others have mentioned is that you may struggle with moving back as you will get used to the income and lifestyle.
BB x