Green’s Dictionary of Slang

to have sexual intercourse; sometimes ext. as have it away together.

1966		T. Keyes All Night Stand 12: He could chat up a chick and he didn’t mind having it away with some man either.
1977		P. Reading ‘Sonnet’ in Nothing For Anyone 10: Why can’t you do […] the real nitty-gritty, ‘I regret / having had you back when you’d had it away / with that bitch!’.
1982		P. Bailey Eng. Madam 99: While she was working for him she was having it away with every single one of his trainee barbers.
1999		G. Seal Lingo 126: The slang terms available to describe sexual intercourse are almost as numerous as those related to alcohol and include: [...] have it away (also a Britishism), and so on and on.
2001		K. Waterhouse Soho 172: Could be Jenny Wise and the randy young telly bloke having it away, too impatient to go round to her place.
2006		in D. Seabrook Jack of Jumps (2007) 302: He said something about having had it away with her.