The constraint in the flour production process for retail use is the packing; there’s plenty of flour around but the vast majority of it is produced in bulk for industrial scale baking. With lots of people at home, and baking a popular activity, that’s why there’s so little available on the shelves.
There's at least 3 historic watermills - two, I'm told, in Warwickshire and the other quite close to me - that have the grain and have gone back into flour production for the local market.
The town where I currently live is only 12,000 population but has been blessed - or cursed, depending on your viewpoint - with having five big supermarkets; Waitrose (the largest), Asda, Aldi, Lidl and Iceland. Waitrose were a bit slow off the mark with social distancing measures, and it still can be a bit of a free-for-all inside the store because there is no one-way system. Instead of going in every day for a £10 shop and a free newspaper, I'm now going once a fortnight for a mega-shop, and I've been able to source 95% of the stuff I need there; for example, you can usually get flour now, seeing as the panic-buying phase is over. Iceland (which provides a much more relaxed and organised shopping experience, but seems to have huge problems in their supply chain getting stuff into the store) is my back-up for the remaining 5%.
I've been going to Waitrose late afternoon or early evening and there's still a bit of queuing outside. Last night, around 15 punters were patiently waiting and it was 10 minutes or so until I got in. Not the end of the world.
My current grouse with them is why oh why do they still insist on moving things around to different shelves /aisles

? You have a long list, you know the store layout, but it suddenly dawns on you the chipped tomato tins ain't where they're supposed to be. So you're forced to search and/or backtrack. No wonder it's chaos at times in there ....