Like Unliver's Marmite, Walkers crisps are made in the UK using ingredients sourced in the UK and like Unilever, Pepsico are trying to profiteer at our expense. Pepsico report their profits in dollars and the pound is weak against the dollar. This doesn't make Walkers crisps any more expensive to produce, it just means they get less dollars when they transfer their profits to their parent company in America.
