I saw this on Mastodon posted by @infobeautiful@vis.social and figured that it was appropriate for this community and absolutely not controversial in any way shape or form.
Sausage gravy uses pan drippings from cooking ground breakfast sausage to create a bechamel sauce. It's usually then flavored with black pepper. Breakfast sausage is also often flavored with sage. From what I understand American-style breakfast sausage isn't really a thing in the UK so it might be difficult to picture the flavor profile.