Don't buy your bread from a supermarket (in fact, buy as little as you can from a supermarket). I can't speak for the entire UK, but in London and south east in general (essex, kent), you can find very good sourdough. The UK also has some very good mills and we're spoilt for flour (if you bake at home).
I'm in the north east, and not even in a major city, so the selection is very limited. Like, I could drive 15 minutes to the nearest "local" bakery but then like I said, anything that isn't just your normal toast bread is stupid money per loaf because it's "artisinal".
>>The UK also has some very good mills and we're spoilt for flour
That's interesting, because I find the exact opposite - not in terms of quality of course, but in terms of variety. I find that usually you only have your normal plain, then self-raising, then maybe two types of wholemeal....and that's it. In Poland where I'm from you'd get at least 10 different types of normal white flour in every store, based on how finely they are ground and how pure they are(denoted as "type" from 450 to 2000), and then you have flour from Poznan, from Warsaw, from Wroclaw etc etc - all found in normal stores. In UK it's just "plain". Tesco's own brand or maybe one other "named" brand.
> In Poland where I'm from you'd get at least 10 different types of normal white flour in every store, based on how finely they are ground and how pure they are(denoted as "type" from 450 to 2000)
Okay that's impressive. I buy my flour from Shipton Mill online[1] because, as you noted, supermarkets only carry commercial flour with very little variety, if any. Worse even, you don't get much information on the packaging about how coarse/fine it is, many times.
Side note: I'm also not originally from the UK and had also noticed the lack of bakeries, or specialist shops here. Even smaller towns have your typical Tesco/Sainsbury and few if any local baker, butcher, etc. Where I'm from, a neighbourhood without a butcher, bakery, fishmonger, etc would be considered a bit strange. I don't know why but it seems like the assassination of British culture is almost complete. I don't know if it was consumers that drove it or if the big supermarkets imposed it. They've made the local merchant almost extinct. And don't get me started on food culture (most brits I know don't regularly prepare their own meals, and when they do, they're rarely traditional home cooking outside of the typical sunday roast with the family, or christmas dinner. It's a sad loss in a way.