X-Ray, 4 GCE or 5, filter indeed and more than you might think.
I remember an FE at Lindholme in 1963 going on about it.
Read the Daily Mirror - with 4 O-levels you could become a sgt aircrew.
Read the Daily Telegraph - with 5 O-levels you could become an officer.
In 1962 we had two airmen, one a call A/Sgt and the other a Sgt Sig, both retraining as navigator s and both passed out as Sgt Nav in 1963 but even then the writing was on the wall. Their postings were limited to the kipper fleet IIRC.