Microsoft's UK research centre is in Cambridge, a city notorious for its high housing costs. Google's UK headquarters is in London, and its largest engineering office in Europe is in Zurich - both very expensive cities. Across the pond, tech companies consistently choose to base themselves in Silicon Valley, Boston, Seattle, or NYC; not in Des Moines, Iowa.
Skilled staff want to live in interesting places. Rents would be cheaper in Manchester, but GCHQ would struggle to recruit there - look at the fuss the BBC's employees are making about relocating to Salford.