All wartime RAF recruits were in the reserve (RAFVR) (RAF Volunteer Reserve).
Sometimes people were offered the choice of joining immediately (as a lowly airman) to do menial tasks on a station or just remaining as a civvy (with a VR badge) until a training place materialised.
Sometimes there were long waiting times to be allocated a place in the training pipeline/sausage machine.
Bomber Command was not volunteer only - other than the fact that all aircrew were 'volunteers' to join the RAF,once in the training pipeline the u/t aircrew were placed either where the RAF thought they were the most suited or perhaps more importantly - where they were required to fill squadron vacancies/requirements.
Wittering might have been a 'holding posting' - in the RAF when awaiting the next training course - all service personnel had to be posted 'somewhere' and be on the 'books' of a station or unit,so if there was a (say) 3 month wait for a place on the next required course/training then the serviceman would be posted wherever the RAF decided to post them,it would not matter which command the station/unit was part of.Once a training/course place became available then the serviceman would be posted to that course.