What is a yard?
A yard is 3 feet or 36 inches. In the US you mostly see it on football fields, which run 100 yards between the goal lines, and in the UK short road distances are still signed in yards.
The yard is traditionally the distance from the nose of King Henry I of England to the thumb of his outstretched arm, though its length varied from place to place for centuries. It was only fixed internationally in 1959, at exactly 0.9144 meters.
Nearly every other country uses the meter instead. In the US, bulk soil and mulch are still sold by the cubic yard, which is 27 cubic feet.
What is a UK nautical league?
A UK nautical league is three UK nautical miles, ie 18,240 feet or 5,559.552 meters, which rounds to 5.56 kilometers. The international nautical league is slightly shorter at 5,556 meters.
Don't confuse it with the land league, which is three statute miles. The nautical league is three nautical miles, about 3.45 statute miles, so it's roughly 15% longer.
Britain's territorial waters once reached one marine league – three nautical miles – from the coast. The limit has been 12 nautical miles, or four leagues, since 1987.