Eggs are good, and healthy. You can get 120g packets of smoked salmon trimmings for under a pound at Sainsburys, so a six-pack of eggs and one pack is two or three meals, depending on how hungry you are. Bacon can be surprisingly low in fat if you trim it carefully. With this in mind, also check out gammon steaks.
Oatmeal is cheap, and a nice breakfast - you can liven it up with powdered cinnamon, or sultanas.
Pound shops can be good for things like tinned tuna. Check the Best Before dates before stocking up. Also Aldi and Lidl's if you have then near you.
Beans, or course! Even cheap tinned baked beans can be nice, shop around until you find decent ones cheap. And dry beans are very cheap, and you can make nice stews with them. Or tinned beans like chick peas and kidney beans, mixed with chopped onion and low-cal salad dressing to make a bean salad - I add chopped celery and cherry tomatoes, and sometimes poached chicken. Lentils are also good, look up dhal recipes; experimenting with spices can keep things interesting.
Check the freezer section for cheap frozen chicken and fish. Sainsburys do very cheap packets of white fish and small prawns, and I expect other supermarkets do too.
Keep an eye on special offers. You might find the moneysavingexpert.com site useful for keeping track of offers.
Downshift brands - if you buy anything brand name, experiment with a cheaper own-brand substitute. Some might not do, but sometimes you can find good replacements.
Also, keep an eye out for when your local supermarket reduces things - I get a lot of vegetables reduced to clear around eight at my local supermarket. A big saving.
Hope some of this is helpful.
