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At a guess...
Moving jobs may necessitate adding considerable time on to a commute from outside London, and likely less if you're already in London (even isolated). You get less choice about the first section of your commute if you're outside London.
Of course you may choose not to go after those jobs, but that means your limiting your possible future employment options in doing so.
A serious question after a lot of the talk about different areas on here ... is the reason people would rather stay in London but 'isolated' rather than with a short* commute but not in London a result of train fare prices or something else?
* around 20-30 minutes I'd say