It is peobably the case that UK supermarkets own brand tomatoes are canned here in the same factories from a blend of UK grown and imported tomatoes.
However, the best plum tomatoes are grown in places like southern Italy - where the levels of sunshine and the unique soil conditions produce the optimum flavour, sweetness and acidity not attainable in more northern locations. They are expensive to produce which is reflected in their higher prices.
Napolina tomatoes are grown in southern Italy and canned in their own factory in Puglia.
However - they are not the best you can buy. Universally used by chefs are a tomato variety named San Marzano. They are in another league, but far too expensive for my use. Waitrose sell Mutti San Marzano, a 400g tin costs £2.00. I have tried a tin and they really are special - but not special enough for my use - so Napolina is a very satisfactory compromise.