Surprisingly, this saying is not British!
An ancient Arabian proverb says “An army of sheep led by a lion would defeat an army of lions led by a sheep”
Chabrias said that “an army of deer commanded by a lion is more to be feared than an army of lions commanded by a deer”
During the Crimean War a letter was reportedly sent home by a British soldier quoting a Russian officer who had said that British soldiers were “lions commanded by donkeys”
Karl Marx and Frederick Engels used the phrase on September 27, 1855, in an article published in Neue Oder-Zeitung, No. 457 (October 1, 1855), on the British military’s strategic mistakes and failings during the fall of Sevastopol.
The joke making the rounds of the Russian army, that “L’armée anglaise est une armée des lions, commandee par des ânes“
Plutarch; Morgan, Matthew (1718). Plutarch’s Morals. London: W. Taylor. p. 204.
Plutarch, Regum et imperatorum apophthegmata 47.3
Tyrrell, Henry (1855). The History of the War with Russia: Giving Full Details of the Operations of the Allied Armies, Volumes 1-3. p. 256.
Marx, K & Engels, F (1980). The Reports of Generals Simpson, Pelissier and Niel. Collected Works, Volume 14: Progress Publishers, Moscow. p. 542.