[firmgently]

"a long aisle with all the bread London had to offer." - to be fair it sounds like that's what a single shop had to offer. But it is true that the majority of bread sold in the UK is terrible (made cheaply following the Chorleywood process as mentioned in a sibling comment). The bread in your picture looks like some kind of pumpernickel or rye - most supermarkets have that - the German ones (Lidl/Aldi) do for sure.

London isn't culturally homogenised (although certain central parts are a bit, especially the touristy areas). You can find all kinds of exotic and amazing foods in London - in part because there are many many different immigrant communities scattered around the various boroughs. But you aren't going to find them in one day, or in a supermarket or shopping mall, so it's not a quick or easy thing for someone on a short visit. Even living there for years a lot of people end up getting intimate with what's available in 2 or 3 boroughs and miss out on everything else.