Re: Why Berlin?
Start up areas tend to benefit from network effects: once somewhere is established it attracts other parts of the eco-system, which help employees move between companies or create their own. Berlin's attraction was not just cheap rents, but also the ability to attract quality programmers and engineers from Poland and elsewhere.
That said, start ups also tend to congregate around other existing business areas, which is why Düsseldorf also has its fair share (telecoms, fintech, etct.) as do Munich and Frankfurt. London's scene is almost entirely dependent upon money from the investment sector and if that moves, it will. But don't expect any major announcements until the necessary deals have been done.