In article , Rodger Whitlock
writes

Regrettably,
the native squirrel is very territorial, and experts think that
the gray squirrels (not very territorial) will drive it out of a
lot of its range in time. It may be that that's why your British
red squirrels are scarce now - competition from the grays.

The competition is from humans. British red squirrels are scarce because
they are unable to thrive well in modern rural (and urban) conditions.
In this area they had become very scarce long before greys arrived.
--
Alan & Joan Gould - North Lincs.