hummingrog, I've just got all my little ducks in a row, EFTS, SFTS, AFU, OTU, and obligingly you have provided an interesting example of the penultimate species. Interesting because though Southrop was just the other side of the A417 Fairford-Lechlade Road, I had never heard of it despite having once been stationed at RAF Fairford, and interesting because you say it was 3(P) AFU. The (P) I presume meant pilot, begging the question of what other types of AFU there might be...
Encouraged by Danny's ever helpful post, and intrigued by yours I turned up this:-
Flying Training Schools_P
So it seems that the original UK FTS's became SFTS's wef 3Sep1939 and then redesignated AFU's (P) in 1942, before completing the cycle post war of SFTS and then back to FTS. So your father's modest pied-a-terre at Southrop was one of many 3FTS locations in its various incarnations, starting in 1920 at Scopwick (Digby), then later Spitalgate, South Cerney, Stormy Down, Bibury, Long Newton, Wanborough, Feltwell, Leeming (JPs), Manby, and Cranwell (Tutor). A long and distinguished history of pilot training, and almost as old as the RAF itself. Achieve would seem to be a fitting and inspiring motto!