You won't find anything at all like that here. In over 10 years, I've never come across any real selection of British products in Buenos Aires. The only things I've found are:
HP sauce (in some specialist wine shops and the buque bus duty free)
Twinnings tea (used to be able to get this at some larger supermarkets...I've seen some musty looking packets in the windows of specialist wine shops)
Colmans mustard powder (in half kilo boxes...found very very rarely in China Town)
Marmite, half decent tea, cadburys that doesn't taste like plastic, biscuits not made with cow fat, salt and vinegar crisps, etc, etc, etc will have to be brought over with you...
Came across some stuff in a curiosity shop in Mar de Ajo but was an arm and a leg job so didn't bother.One thing they had was malt vinegar!!! a must for fish n chip wallahs.Special brew is another one would tend to use as a sleeping draft.