The base of the Pimms cup is an apertif called Pimms Cup No. 1 gets its name from James Pimm, who operated an oyster bar in London in the 1840's, and mixed his digestive tonic with gin, herbs and quinine and other never revealed ingredients. I suspect that it contains some of the aromatic ingredients found in aromatic bitters like Angostura. Angostura would have just arrived on the English market around the time that Pimm was concocting his tonic. Pimm's drink was classified as a gin sling and could have been the first English cocktail although it was probably served without ice. The early definition of the cocktail was a "bittered sling". Iced cocktails didn't make their appearance in proper British society until the end of the 19th century and weren't popular until the First World War with the invasion of all the American service men and their penchant for iced drinks.
James Pimm with the help of some well heeled investors from among his clientele produced Pimms # 1 cup commercially until he died. Eventually the brand name was extended to include Pimms # 2 based on whisky, # 3 based on brandy, # 4 based on rum, # 5 based on rye, and #6 based on vodka. Pimms # 1 is available in the United States and Pimms # 1 and 2 are still available in England. In England Pimms Cup is served with lemonade, that is, the British style of lemonade, comparable to our lemon-lime soda. I have made my Pimms Cup for years with real lemonade topped with a splash of soda or 7UP, I find the fresh lemonade version much more refreshing and probably closer to the original 1840's drink when lemon lime soda didn't exist. With the Wimbledon Tennis Tournament coming up, Pimms season is here. According to Nigel Watson's Tennis School, keepers of the "Wimblers Stats", 40,000 pints of Pimms No. 1 Cup were served at Wimbledon last year. The true way to serve Pimms No. 1 cup according to the traditionalists is topped with Champagne in the Royal Pimms Cup.
http://www.bevaccess.com/bb_pimms_cup.html