Originally Posted by mickeys
A better choice might be the **FREE** K-Type "Keep Calm and Carry On" typeface, available at http://www.k-type.com/?p=2156
The original was hand-drawn.
P22 Underground has a similar look, but is used on London Underground signage.
And Gill Sans is a kinda rip-off of Johnson's Underground font (something Eric Gill admitted) It was designed because London Underground held the licence for Johnson Underground and wouldn't allow others to use it. The poster, although probably hand-drawn, has very similar characteristics to Gill and Johnson Underground and one of them was probably a reference point for the designer of the poster.
Good link Mickeys
, I should've known some canny designer would design a font of the poster.