Born out by experience
I'm working at a start-up in Cambridge where we pay mid-range salaries and need experienced people, and the majority of the software team are non-UK citizens, and the majority of the CVs we get sent by agencies for the open positions we have are also from non-UK citizens. We're in a city where the market for experienced software engineers is competitive, so one reason may be that the good UK engineers go to the bigger companies with the deeper pockets, but there doesn't seem to be the pool of talent in this country that there once was.