Try spotted dick.
Going by the name alone, that dish doesn't sound like anything you'd want to eat in public - and the spotted variety not even in private. But spotted dick is actually just a roll of baked pudding that has pieces of dried fruit in it. It is commonly served at restaurants and pubs. The dough is made from a mixture of flour, milk, caster sugar, suet, dried fruit, flavorings and baking powder. (What is suet? It's the raw, hard fat of beef or mutton found around the loins and kidneys.)
Why does it have a name like that? Well, the dick is a corruption of the word pudding, pronounced puddink in some dialects and the pieces of dried inside the roll fruit look like spots on its surface.
We can talk about the famous "fish & chips": a fish fillet and fries, that used to eat the workers in newsprint (for the record, you should know that selling fish & chips In newspaper was forbidden because the ink of the paper passed in the food through the used frying oil, so we ate the ink: but it was unfortunately very bad for the health!).
The Sunday Roast / Sunday roast is also a traditional dish: it is the equivalent of stew or beef bourguignon. You can enjoy roast meat in this dish (the most traditional is beef but you can also eat chicken or mutton!), Potatoes, various vegetables, gravy sauce (often made with cooked meat juice and vegetables) and one or more Yorkshire Pudding (s).
You should also know that a dish of Indian origin is nowadays seen as a national dish: Chicken Tikka Masala, a dish made with chicken in curry sauce. It was invented by a person of Indian origin who came to settle in the United Kingdom. This shows the mix of cultures found in the UK.
Fish and chips, that's a classic of English cuisine. Perhaps not the most traditional, but in recent years this dish has become something unique in London and whether or not you should try when you visit.
Another very personal recommendation and that goes well with the Fish and chips is the Yorkshire pudding, an exquisite dessert and very traditional in the English gastronomy.
When I think of the english I always think of tea and crumpets or fish and chips. Not sure why but those are rust how it is. Unless you count the quote from the film Pulp Fiction. "A royal with cheese". lol