But what's the font?
I've been arguing the toss for a couple of days now with fellow typography nerds, and almost had it nailed as a version of Gill Sans, but on closer inspection it clearly isn't . Neither is it Johnston, nor the great P22 Underground typeface.
All I know is that the original commission for the poster stipulated that it should be in a "special and handsome typeface" to make it difficult for the enemy to counterfeit it. As such, it appears to be a 'one-off'. Some unknown graphic designer missed his or her chance to make a mark on history with 'Blenkinsop Sans' or 'Fothergill Futura'.