My local Waitrose has had bread flour of one sort or another the past couple of weeks when I've been in - though I do go early in the day. It's been dire in Tesco, though.
However, when I was running short before that I managed to order a 16kg bag online direct from a mill. Some have closed their online sales entirely, but some are open for a short time each day, so you may need to keep trying places repeatedly until you catch them. Try searching for "wholemeal bread flour UK" (or whatever kind you want) and see what comes up. In my case, the cost of flour plus delivery came to a bit more per kg than I'd have paid locally, but not massively so - certainly not as much as the rip-off merchants on eBay where you can pay £10 or more for a single 1.5kg bag.
It's a similar story with yeast, though I haven't seen any at all in the supermarket for weeks. I usually use Allinsons Easy Bake (for a machine), and currently you can get a 100g tin on eBay for about £10 instead of the usual £1.20 or so (from memory). Anyway, a couple of weeks ago I came across another brand (Fermipan), with sellers on Amazon and eBay, and as the reviews were quite good I thought I'd try it - 500g for a bit under £10, but smaller sizes are also available. I haven't needed to open it yet, so I hope it's OK - the flour I got was very good, though.