There's lots of choices in London for ways to fill your tummy. You can spend a lot or a little or somewhere in the middle.
Pubs are the most obvious and popular choice. Pub grub is usually plentiful and filling and reasonably priced. But these days, there are also Gastro-pubs which means an upscaled menu and higher prices. One chain of pubs that I tend to frequent is the Weatherspoon's pubs. they are quite often in older historic buildings and the menus usually have a good variety with low prices. They also have a small number of items that are discounted or are two for one offers. Those will be standard things like Bangers and Mash, a burger or fish and chips. For the most part, their food isn't bad and sometimes quite good. Once in awhile it's only mediocre but that's pretty much true of any place.
There are hundreds of coffee shops and cafes around, and you can usually get pretty good meals for reasonable prices there. It's sometimes worth walking down a side street off a main one to find cafes and restaurants that are slightly off the beaten track but which have great food at good prices. Chinese and Indian restaurants also have lots of value for money, especially it it's one of the many buffet restaurants. There's loads of these in areas like Soho's Chinatown, or the east end by Brick Lane.
Restaurants will usually always have a menu displayed outside so you can see ahead of time what they offer and the cost. There are often dinner or lunch specials or set menu prices which can also be good value. Pubs also may do a Sunday roast dinner special which I always find to be filling and tasty.
I tend to avoid for the most part the big fast food joints. Food is usually overpriced and mediocre. Having said that, there are chain restaurants like Pizza Express or the Gourmet Burger Kitchen that have fast and tasty food.
Some churches have cafes that are small but afford good food at reasonable prices, usually only open around lunchtime.
Restaurants that I've particularly enjoyed for a really nice meal include Greek food at Kalamara's in the Bayswater-Queensway area, Andrea Doria for Italian in SoHo, Salieri's for post or pre theatre on the Strand, Loch Fyne for seafood in Covent Garden (and elsehwere, it's a national chain), and the historic Lamb and Flag pub in a hidden corner of Covent Garden.
Another thing that's useful is to get takeout food such as sandwiches and fruit at a Tesco Metro or Marks and Spencer food store or the Pret chain of cafes and take it for a picnic or just a light meal in your hotel room. Boots the drugstore also has premade sandwiches and snacks.
Fish and Chips, you say? Well most of the places in the touristy areas are locally known to be overpriced and not that good. Some pubs do it well but for good fish and chips, wait until you're outside London on a day trip somewhere. You'll probably have better luck. That said, sometimes if you are in the outer areas of London such as Notting Hill, Dulwich, Hampstead, or Camden, you might find a good "chippie" on a side street or a good feed of fish and chips in a pub.