Disclaimer: All of the below comes with the obvious caveat that it has to be made by someone who knows what they're doing. Any country's cuisine is shit if you're eating at a shit restaurant. That's not about your choice of meal, that's your choice of venue.

Pies. Britain has basically perfected the savory pie. Steak and kidney, steak and onion, scotch pie, shepherd's pie, cottage pie, fisherman's pie... British savory pies are an explosion of flavour. Pair with a good stout or porter; the kind of beer that still looks black even when you hold it up to a light.
Good fish & chips is one of the most perfect meals ever created. Keep in mind that the condiments matter; you need the acidity and richness of a good malt vinegar to cut through that fattiness. Never get fish & chips in a pub or restaurant. Go to a proper chippy, preferably in a coastal town.
Stews are a British classic. Try a real Lancashire hotpot sometime, with pickled red cabbage and mashed potatoes. Absolutely stunning.
Get your ass down to a high quality carvery and try a roast dinner. Lamb or beef are the best bet, but chicken and pork are good too. The true test of a roast dinner is the quality of the gravy. It should be not too thick, and full of savory meaty flavour, not doughy or floury in taste. Also look for vegetables that have some colour on them, not just boiled. Roast lamb leg with a good mint sauce is a thing of beauty. Pair with red wine for lamb or beef, white for pork or chicken.
Haggis is a flavour explosion. The real thing, no plastic wrapped forgeries, served with the traditional sides of tatties and neeps, and a glass of really good single malt whisky.
Straight up, one of the best meals I've ever had was bangers and mash. I was at a high end London restaurant - I forget the name, but John Lennon used to eat there pretty regularly - and it was incredible. The potatoes were the perfect texture with just the right amount of salt and butter. The sausages were made in house, beautifully seasoned and cooked to perfection, and the gravy was stunning. It's a very simple meal, but simple done right isn't easy. In a simple meal there's nothing to hide behind. Every part has to be perfect.
Desserts. British desserts are phenomenal. Eton mess, spotted dick, and the absolute king of desserts, sticky toffee pudding served with thick cream. Unbelievably decadent.