Sir Hugh Casson was a past President of the RA, 1976-84 and founded both the Friends of RA in 1977 and its American counterpart in 1983 - so it owes him a lot.
This exhibition (why in 2013 it doesn't celebrate anything?) is slight comprising just two rooms mainly of sketches and letters. The former draw heavily on the work of Edward Ardizzone.
I liked Augustus John's studio (1935) which Casson had a hand in more for its genuflection to Le Corbusier than anything else.