Starlings

I've seen flocks like this in Kentucky and Indiana. In the U.S. the roost locations for these immense gatherings often become contaminated with a fungus called Histoplasma capsulatum, which can cause human disease. Whether this is true in the U.K. or not, I don't know, because it also depends on characteristics of the underlying soil.

Here's an Audubon Magazine article about flocking behavior.