Green’s Dictionary of Slang

collar (and cuff) n.

a homosexual.

1934		P. Allingham Cheapjack 24: He tried on my silk hat and the others agreed that he looked ‘a right collar and cuff’ [...] ‘No one ain’t a collar ’ere,’ he said. ‘And if you think a gent’s titfer can make me look like a sissy I’ll give anyone a sock in the lug.’.
1960		J. Franklyn Dict. of Rhy. Sl.
1978		Maledicta II:1+2 (Summer/Winter) 118: Elsewhere Aylwin lists a few more ‘Vulgarities’: [...] queer (Brighton Pier, though often in the sense of ‘peculiar’ rather than what Aylwin calls a ‘puff’, collar and cuff).