Not officially but unofficially Beans on Toast is most recognizable other than Fish and Chips or and Full English Breakfast

Official National Dishes are always suspect. For example, the US has Turkey in general, and Apple Pie, which is an English dish

Realistically, Pies should be the national dish of the UK (Mince Pie, Shepherds Pie, Pork Pie, Pasty, Fish Pie, Many Dessert Pies) https://en.m.wikipedia.org/wiki/Category:British_pies