I think I have to agree with Ross.
Personally, I'd be looking to avoid band-aid solutions to 'pay the bills', and the inherent constant uprooting that it's likely to breed. First and foremost, however, I'd make sure I'd allotted enough working capital in my business plan to allow for the time and effort that building up a loyal clientele can take in the coffee game.
Again, I'd love to see a lot more places to get a great cup of coffee in London, but Soho (and much less Berwick Street) is not one of them.
All the best, all the same.
T.