Melting In Your Mouth

This slogan was first used in the USA, in 1949. But M&Ms weren't introduced in the UK (or the rest of Europe and elsewhere) until the 1980s. The slogan was first used in the UK in 1967, for Treets. I refer to Wikipedia for corroboration, but I can remember it myself.

Readers of a similar vintage to myself will remember that there were eventually three varieties of Treets: Peanut (the original), Chocolate, and Toffee. Chocolate Treets were rebranded as Minstrels (now known as Galaxy Minstrels), and the same slogan was also used for these. Peanut Treets were discontinued when M&Ms were introduced in the UK; Toffee Treets were later sold as Relays, before being dropped altogether.

All three varieties of Treets were also available in Revels, but without the candied sugar coating.

And one more thing: I believe it was your hand, singular – not hands plural – that they didn't melt in. Wikipedia agrees on its Treets and M&Ms pages, but gives the plural version on its Minstrels page!