Re: Old typewriter
"what methods did the British have to hide their communication from the Germans during the war"
Obfuscation and contradictory messages. Those in the know would know which was valid, to others it would look like nobody was in charge.
Subtle non verbal message passing, as has already been mentioned, playing certain pieces of music at certain times of the day.
Then the Americans brought in the ultimate encryption device - a dude that spoke Navajo.