Many people have worried about eating in London and in the U.K. in general because they have heard that British food is horrible. That used to be true, at least to some degree, but London is a huge city and incredibly diverse, so food is diverse as well. Think about the cooking shows you have watched lately on TV. How many of them come from British cooks? Gordon Ramsey ring a bell? And The Great British Bake Off?
"Classic" British food can still be found everywhere in the United Kingdom, with the term "classic" referring to things like fish and chips, bangers and mash, sausage rolls, shepherd's pie, hot pots, Cornish pasties, and Sunday Roast. Most of these traditional foods tended to be a bit under-spiced, and rather bland, but you can still find them in pubs and local restaurants. And almost every museum, attraction, or art gallery will have a tea room, which serves tea, coffee, soft drinks, and a wide variety of what the English do best--cakes and pastries!! (If you have never had Victoria Sponge cake, you are missing out on one of life's greatest pleasures! Ditto for freshly baked scones filled with strawberry jam and a huge clump of clotted cream!)
On the other hand, London and most large cities have been overrun in the past couple of decades with just about every fast food restaurant you can think of: McDonald's, Burger King, KFC, Pizza Hut, Subway, Wendy's, Starbucks, and Five Guys. These can easily be found in train stations or just outside of train stations.
Today, London has 8.8 million residents, with almost half of them having been born in other countries! And when people come, so does their food, so you can find almost any kind of food in London. You might need to ask around for some of the best which tend to be outside the tourist areas, but they will be worth the effort.
Also, the British office workers and shoppers rely heavily on "grab and go" food, which tends to be fresher and more widely available than in the U.S. By take-out, I am mostly referring to pre-made sandwiches, soups, salads, and deserts available in refrigerator cabinets in convenience stores, department stores, grocery stores, and dedicated take-out stores such as Pret a Manger, which is an excellent all-organic and freshly made food. You can hardly walk anywhere in London without seeing one of these shops.
Of the above, my favorite is Pret because they donate all sandwiches, salads, and most other unsold things every evening after closing. The recipients are various rescue charities and food banks. Employees arrive very early every morning to make the day's replacements, so you can be sure everything you buy there is fresh. In addition, because they specialize in organic food, if you find something with ham in it, the meat comes from a big sliced whole ham, not a deli that slices preserved hams. Ditto for a chicken sandwich containing only freshly cooked chicken breasts, not chicken deli pressed and formed meat.
Another place I relied on heavily when I traveled in London was M&S, or Marks & Spencer, which is a major clothing store. Sound strange? Just walk past the clothing racks to the back of the store, and you will find a grocery store focusing heavily of pre-made sandwiches, salads, and other foods, plus things like cookies, chocolate, coffee, tea, and other things you might want to take to the office or home for a treat.
Also, if you are grabbing a sandwich or salad and a drink in a supermarket or clothing store, ask about what they call a "meal deal." A meal deal is fairly new but has spread like crazy. Basically, it is a main, snack, and a drink. The "main is likely to be a sandwich or a salad" and the snack can be something sweet like a piece of cake or pastry to a chocolate bar. It can also be something salty like chips or pretzel. They cost about £4-5, so you save quite a bit on these. Pret just started its own meal deal, and it costs between £6-7, but their food is all organic, and worth it. They meal deal includes "any bread-based sandwich, crisps, and a drink."
You will not go hungry in London, but one tip is to choose a hotel in or very near a commercial area with lots of shops. The first year I went there, I picked a hotel in an area with a lot of high-rise office buildings, and everything closed at 5 pm, so I had quite a walk in the dark to find a restaurant or fast food.
In any case, when you can't find what you like, don't forget that the larger train stations are filled with restaurants and shops.
Oh, and the big news is that McDonalds in the UK now serves breakfast, but be aware that almost all of their breakfast sandwiches have mayonnaise on them, even things like Egg McMuffins, so you might want to change the condiments.
A Warning: A big item that people from the U.S. and the British strongly disagree on is the necessity for ice in drinks. Basically, Brits do not like ice in drinks and feel it waters them down too much. Even if you ask for ice in your drink, you will probably get no more than one or two ice cubes. I remember once getting a Dixie kitchen cup with one ice cube in it to accompany my 16 ounce Pepsi! I should have taken a photo.
And do read labels of drinks. Many of them contain artificial sweeteners even if they are NOT labeled as diet drinks. I wrote about this here in third and fourth paragraphs: https://2025humongousenglandwalesadventure.blogspot.com/2025/08/723-food-restaurants-in-uk.html