> As an example, interns at Google in the US earn about $7000 a month, while in London they get about £2000. That's because of market rates in London are far lower than in the US. Ideally, tech companies in Silicon Valley would really love it if they could have the same employees as they do now for half the price.

Just speculating, but I imagine for Google the real estate cost of moving their headquarters to London would far outweigh the small amount they'd save on labor.