What and where is M&S?
Marks & Spencer, the most British of shops. Although now I look, I'm amazed to see that despite having shops in Legnica and Koszalin, they have no store in Krakow.
However, Kuchnia Swiata are in Galeria Krakowska at ul. Pawia 5 (-1 level). I'm almost completely sure that I've seen corned beef one of their Warsaw stores, but it will not be cheap.
If you strike out at Kuchnia Swiata, or if you want enough cans to make the 100zl delivery charge a sensible investment:
britishcornershop.co.uk/product.asp?id=160