When Pret launched, your options were a packet sandwich from Boots or something, which would be stale sliced bread with a single layer of tasteless waxy cheese, or something from a sandwich shop, where the choices are (1) tuna mayonnaise that has been there since yesterday (2) a cheese bun wrapped in cling film and kept in a fridge (3) a (one piece of incinerated) bacon, (limp) lettuce, and (tasteless, but mercifully thinly sliced) tomato.
Things are a bit better now, but to be honest, still terrible. You can wander around New York or Madrid and find a random sandwich place on a street corner that would be in the top 10 in London.
My theory is that this a manifestation of a broader cultural weakness of (most) British people, which is that they can't imagine that a better world is possible. Nobody looks at that "famous ham and cheese" and thinks, "okay, but what if there were five slices of each of those, and the bread tasted of something, and there was lashings of a delicious wholegrain mustard dressing?". Because they don't imagine it, they don't demand it, and Pret keeps on churning out abysmal lunches.