There are direct and indirect proof of your plans to live outside Greater London.
Empowering North was an option, active till July 5 2018, now you can live everywhere in the UK except the capital in order to apply for Fast Track Option.
Regarding direct proof - you need to write in your Personal Statement about this point and this is "must have" option.
Indirectly you can provide some information in the supporting documents, that will show your intent to live outside London.
In my blog you can find more information about this and other issues of Stage 1.
And finally - if you will be rejected on Fast Track option it means that you are rejected at all, the Panel will not re-consider your application with an ordinary route.
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