What is bubble and squeak, and why is it part of an English breakfast?

Bubble and squeak is a traditional British potato dish made by frying leftover vegetables, most commonly cabbage and potato, until crisp. It originates from domestic kitchens and the reuse of vegetables from the Sunday roast, which places it firmly within British food tradition. Its role on an English breakfast is to add balance and substance without competing with bacon, sausages, or eggs, supporting the plate rather than dominating it.