I wrote a short bit about sandwiches in the UK in another thread I'd started, and it was suggested I should post it over here too. So here it is, reworded and rearranged a bit.
I was asked if sandwich shops and delis are a thing in the UK the way they are in the US. The answer is definitely.
First of addressing what we Brits call a sandwich - usually a cold filling between two slices of bread. Although some hot versions are common - a bacon butty (sandwich) is a common thing to grab for a breakfast on the go - 2-3 strips of back bacon, with either tomato or brown sauce. Another hot sandwich that comes to mind is a salt beef sandwich - what you guys call corned beef (which means something else over here).
But the basic sandwich - meat, or cheese, or egg, or..... - is still very much the lunch of the office worker or the person on the move.
As such every food shop and supermarket will have a sandwich chiller cabinet filled with prepackaged sandwiches, with a variety of bread types and fillings. Everything from a very ordinary cheese and tomato or egg and cress on white bread, to quite deep filled sandwiches on fancy breads. (For example, a popular seasonal sandwich is a Christmas "Turkey and Trimmings" sandwich - turkey, sage and onion stuffing, bacon and sliced sausage, with mayonnaise and cranberry jelly)
Some stores will make them themselves, particularly if they have an in-house bakery; but most are bought in - sandwich making is a big business.
The largest company by far to supply sandwiches to stores is Greencore, who turned over £1.3 billion last year (that is not all sandwiches - they supply something like 600+ million packs of sandwiches in a year. The total market is about 2 billion packs, while it is estimates 8 billion sandwiches a year are made at home.
A step up from the supermarket sandwich, we have many independent bakers still who usually also do sandwiches, except they are more likely to make them to order, and with their own bread.
Then also many cafes will offer sandwiches - also self-made, on the menu along side hot food.
In between the last two would be the tea room - a kind of cafe, but somewhere you would go for afternoon tea (or morning coffee), offering a small spread of sandwiches, small cakes or possibly scones, jam and cream. Perhaps more a social event than a meal.
Then you have the actual sandwich chains. A big chain here is "Pret" (full name "Pret a Manger") doing hot and cold drinks, snacks and sandwiches and wraps. Although a chain, a Pret sandwich is a bit special, a step up from a supermarket sandwich - Chicken, Avacado and Basil, Breakfast Egg and Bacon, Italian Prosciutto with salad - you get the idea - in a sandwich, wrap, 6inch roll or footlong.
We also have the Subway chain, which many of you will be familiar with.
And then finally you have the burger joints, which we don't consider sandwiches, but my American friends refer to them as, so I'll include them.
But probably, more sandwiches are sold from the supermarkets than all the others, I would guess.