Probably the most numerous would have been the very early Airborne Interception (AI) radar operators,the Battle of Britain Memorial (BBM.org) website has many AC2/AC1/LAC operators who flew without Flying Badge or NCO rank - later the system caught up and the survivors who remained as aircrew would have become at least Sgt W Op/Radio Op etc until the system settled down to Radio Observer then Navigator.
I have just copy/pasted one individual - there are many names on the BBM list....
Robert Sydney Brown was born on 2nd October 1915 in Gateshead and was educated at Shipcote School and Gateshead Secondary School. He joined the RAF in early June 1940 and after his initial training he volunteered for flying duties.
At the end of June he went to Yatesbury for a short course on airborne radar and on 20th July was posted to 604 Squadron at Gravesend. He had yet to fly in an aircraft.
On the night of 1st November 1940 Brown was in a Blenheim on a fighter patrol. The aircraft went out of control and he and the Air Gunner were ordered to bale out, which they did and landed safely. The pilot regained control of the Blenheim and returned to Middle Wallop.
Brown flew operationally with 604, firstly in Blenheims and then Beaufighters until December 1940, when he was posted back to Yatesbury for another radar course.
In these five months he flew without rank or flying badge and was paid three shillings a day, this including one shilling flying pay.
After completing the radar mechanic course and gaining practical experience, Brown was attached to RAE Farnborough, where he remained until his release from the RAF in February 1946.