Clearly you have never been to Oxford, Bath, York, Edinburgh, Lake District, Cornwall etc etc which are teeming with visitors from the US, China, Japan and many other countries during the summer...
I've been to most of them but teeming isn't a word I'd really use. They're also the most obvious hotspots outside London, I was mostly attempting to make a point about all the 2nd-tier and 3rd-tier locations that are currently very busy with UK staycationers.
We did the North Coast 500 this year (we live in NE Scotland, but obviously aren't going to get anywhere further afield for now!), and it was really difficult to find accommodation because so much was fully booked.