Oh dear, seems to be a serious set of misconceptions here.
All corned beef is NOT imported into the UK tins. Standard 340g cans, which are the most common size stocked by stores in the UK, are packed and labelled in the country of origin. They cannot be packed or relabeled elsewhere due to strict country of origin, USA FDA and EEC regulations.
Typically, a 340g Brazilian corned beef can will bear a circular stamp on the end with the words "Brasil Inspecionada" and a packing/processing station code. That stamp cannot be applied to the can in any other country other than the country of origin. Furthermore, the words "Packed Under Brazilian Government Inspection" will be found somewhere on the label. These words again have a legal standing and obviously this eliminates at a stroke the possibility of Brazilian inspectors inspecting the packing elsewhere in the world!
Bulk corned beef packed in tins for catering use bears the same type of stamps and codes and is indeed sliced by food processors and repackaged for sale in those plastic packs of 4 slices or so in the supermarket deli section. Supermarkets can obviously slice the stuff themselves which is what the OP purchased.
Finally, I've been asked once or twice why tinned corned beef has so much fat on the outside when opened. Having visited a cannery in Brazil a few years ago, I confirmed what I'd prevously known. Corned Beef is a liquified product. It's poured into the cans. The fat rises to the surface in all cases. I still love the stuff all the same!
There's also canned corned beef in the shops that originates from France if you you keep your eyes peeled.