Working Dress - varies as to where you are - or perhaps where you are to be posted to.
See this photo from my album of Sgt Henry Moon, NCO i/c SAS Flight at RAF Gan weloming a visiting RNZAF Bristol Freighter in 1958.
This was standard working dress at Gan - I like the size of his greasy rag!
Mind you he dressed differently when he welcomed the C-in-C to Gan as seen here.