Q. Why are the Brits called limey's?
A. The term "limey" originally applied to British Sailors, who for many centuries suffered and often died from scurvy. A Scottish doctor, James Lind, discovered that eating limes and oranges (a good source of vitamin C) greatly reduced the number of scurvy cases on British navy ships. Lime juice was added to the diet of sailors and thus the nickname 'limeys'. American service men broadened the term to all British during the World Wars.
Source: Harper Dictionary of contemporary Usage by William and Mary Morris,1985. p.358.