In England:
If you want an egg salad sandwich, ask for egg mayo. Otherwise you may get sliced boiled egg with lettuce, tomato and cucumber (hence the "salad" part)....and sometimes pickle (like sweet relish with different veggies)
Same goes for Chicken salad. It wont be mixed with mayo, it'll have the cucumber, tomato, etc with in instead.
If you want a salad for lunch, ask if they have green salad. No Ranch dressing though. Boo.
They also tend to butter the bread before making the sandwich, apparently it is supposed to keep it from going soggy...
Potato salad is also alot different. Not bad, just different. Prepare for it to be on the sweet side.
Cornish Pastys are delicious. You can get them vegetarian as well as classic steak and swede (rutabaga, turnip like veggie)... and also try the sausage rolls or cheese and onion rolls, they are usually sold as pasty shops as well. Delicious!!!
Scotch eggs are one of my favorites, boiled egg wrapped in sausage and breaded and fried.
And if you like Black Coffee, its a good idea to order it black. They mostly serve it with milk already added unless you specify that you want it black.