Interesting question, suppose it must be something native to the UK, so no tomatoes, or potatoes, but what did we eat before 1586?
And did it depend on social standing? And was there a national dish before the railways? So much food was transported on the railways, that the whole way of life changed as a result. And some items have to be from an area to have a name, and some items it is how not where it is made, so Cheddar Cheese can be made anywhere, but not Cheshire Cheese, and some cheeses were made from sheep milk, not cows, and some cows are named because that's where they came from.
But if wild boar, was considered as an English dish, today it would be hard to get the wild boar to start with, forest of dean maybe, but cheese on toast has been claimed as a Welsh dish, but try getting a definitive recipe for it, some want egg in it, some want mustard, and the Sunday meal was a Sunday meal as we could not afford a meal like that every day.
But some of the British foodstuff is known worldwide, everyone knows what Spam is.
But, corned beef came in the main from South America, until we had a bit of a conflict, not a war, just a conflict! And our eating habits have been traditionally altered by the Italians, they brought Lions to the UK, and things like olive oil, what did the Romans do for us?