How much is 100g margarine in UK?

It depends on many factors - such as branded versus supermarket's own. It also varies from store to store. However, the range that ASDA sells range from... 88p for a 500g tub of their 'Smart Price' brand (18p per 100g) up to £3.48 for a 500g tub of Benecol Buttery (70p per 100g).