Tinned beef: which one you want
In a UK supermarket, “tinned beef” usually means one of three things: corned beef, stewed steak in gravy, or minced beef in gravy. The last two are stews already, there to be heated and eaten. Corned beef is the one that behaves like an ingredient. It slices cold, crumbles warm and fries to a dark crust, which is why it is the one this site keeps.
What corned beef is
Cured, cooked beef pressed into a tin. The standard UK tin is 340 g, the tapered one you open with a key. Chill it first if you can: cold, it slides out in one piece and slices cleanly. Warm, it crumbles, which is exactly what you want for a hash.
It keeps for years. USDA FoodKeeper gives canned meat products five years unopened. Once opened it is cooked meat and goes in the fridge.
It is salty. 860 mg of sodium per 100 g is about 2.2 g of salt, well over the 1.5 g per 100 g that the NHS counts as high. A quarter of a tin is roughly 1.8 g of salt, getting on for a third of the 6 g a day adults are advised not to exceed. Leave the salt out of whatever you cook it with. For the longer-term question of eating processed meat, see processed meat, honestly.
Cooking with it
The hash is the reason to own a tin: diced corned beef and potato in a hot pan, left alone long enough to crust before it is turned. See corned beef hash, or the version with sauerkraut, where the acid cuts the fat and salt. Sliced cold, it is a sandwich with pickle or mustard. Fried in slices, it does most of what luncheon meat does.