What is closest to the product referred to as grit in Gardeners' World is poultry or chicken grit, sometimes referred to as cherrystone. It is a rough edged gravel product of about 1/8 to 3/16 of an inch in diameter. I get mine at a local feed store but sometimes nurseries and garden centers will carry a bagged grit as well.

FYI, for a container mix, the perlite is preferable to the grit, as in addition to aeration, it also holds on to resources like water and dilute nutrients, which the grit does not.

It’s sold here as horticultural grit. If you Google that you should see images of what they’re talking about.

I've used chicken grit from Tractor Supply and it works well.