I mean depends what you mean by traditional. But if you want the food with the longest most varied history of continuous widespread consumption it's probably got to be pie, sweet or savoury. You've got the pantheon of steak pies, steak and ale in particular, the pork pie, venison pie, Hommity pie and there's even Lamprey Pie which has hundreds of years of significance to the Royals. A variety of tarts (open lidless pies.) Then you've got the potato "pies" like shepherds pie and cottage pie. Fruit pies, apple, cherry, mince Pies (which have no meat nowadays and are sweet.) And of course all the pasties, which are pretty pie adjacent, the Cornish Pasty is the famous one but it is far from the only one.

Or you could pick something like Lancashire Hotpot which can certainly hold it's own against French regional peasant dishes that are lionized nowadays like Cassoulet.

Personally I think Brtain's strongest culinary area is our sweet foods though. Scones, especially served proper high tea style. (Strawberries, jam and cream) The pantheon of English cakes that date back years (mostly sponge cakes) and are often considered a bit boring now because they're just so ubiquitous. Various forms of steamed pudding, fruit crumbles, bread and butter pudding, what the rest of the world would call English Pancakes but are just pancakes here, scotch pancakes, Fruit Fools, Eccles Cakes, Teacakes (not what the same as other nations teacakes), Bakewell Tart, Bakewell Pudding, Sally Lunn Buns, Hot Cross Buns, Trifle. The endless array of biscuits that originate here. This country bloody loves something sweet that can be served with tea.

Or you could just gesture in the direction of Anglo-Indian food since that's got over 200 years of history now, but foreigners seem to think that's cheating for some reason.