(1) English people have famously bad taste when it comes to food, and most of them are truly willing to eat crap.
(2) Many English people are cash or time poor, and this stuff is everywhere for $cheap.
(3) Grab-and-go style sandwiches in boxes are made using industrial processes in which high initial investment results in small margins per sale which are often maximized at the expense of taste, eg. waterjet cutting, commercial standardized ingredients (eg. heavily processed industrial cheeses and meat slices), etc. As purely commercial operations, they don't care about quality until it limits sales.