Traditionally English, going back a good long way? Depends who you were. If you were poor it's basically going to be a variation on a type of grain and vegetable porridge - pottage. Flavoured with herbs if you're lucky, probably not salt, day in, day out. Enjoy!
Flavoured with alcohol if you're richer - frumenty. (Old timers will remember funtimes on a thread that featured frumenty...)
Seriously though, so many suggestions on this thread aren't traditionally British. Rollmop herrings? Hollandaise sauce??
Kedgeree, god love it, is a product of British Empire and at least indicates contact with curry spices, but I wouldn't describe it as absolutely British.
Going back several centuries, the rich were just as keen to show off with posh imported delicacies as they are today, and a lot of the foods that weren't actually being eaten by peasants were fairly 'exotic' - spiced and sweetened, with fruit and nut pastes used in 'savoury' dishes. Basically stuff that my in-laws, who have no truck with foreign food, would think of as 'fancy foreign mess'.
Whenever I hear people talking about 'proper English food' I think fondly of my PIL's incinerated roast meats, the grey leathery lamb and beef (pink meat is a sign of weakness!), the beef cottage pie with no garlic and not even a whisper of Worcestershire Sauce, the utter lack of seasoning, flavourings or dressings, the insistence on 'good ingredients cooked simply' - and I want to throw myself under a bus. Thank feck I'm not entirely English and was brought up with a huge vat of olive oil in the kitchen, a British mother who had embraced Elizabeth David and others in the 60's, and visits to my abroad family, where we enjoyed food that tasted of something.
If I was taking a visitor from overseas around the locale and wanted to expose them to trad British food, I suppose I'd take them to the local tearooms where the cake is very good, find a cream tea for them, an excellent cheese shop, and perhaps to the coast to enjoy exorbitantly priced but locally caught seafood. Can't guarantee it wouldn't be dripping with garlic butter though!