Marks and Spencer's, commonly abbreviated to M&S is a real world British department store. They are a retail outlet selling everything from fashion to food, and as such appears in Miranda as her go to for prepared dishes.
Series 3
In The Dinner Party, when cooking for herself proves beyond her talents, Miranda goes to M&S to buy salmon terrine and potatoes for the couples dinner party that she is hosting for Mike's father.
She and thee other ladies sing, "Praise M&S Putting food on our table, 'cause cooking drives us crazy, We're busy-slash-lazy, Other upmarket food stores are available!" Then Miranda rallies them with the cry, "Now go, ladies, go. Decant in your own dishes and lie!"