I think if the RAF had kept the F-111K order, they probably would have bought a LOT more than the original 50 plane order.
Remember, the F-111 had very good combat radius, and that would have made it very attractive as a replacement for the RAF V-bomber fleet. In short, the F-111K (and probably an upgraded version named "F-111M") could potentially have taken the place of the Panavia Tornado in addition to replacing the Avro Vulcan for longer-range air strikes well behind the Iron Curtain--in short, the RAF would probably have bought close to 150 planes, in my humble opinion! And I could see Rolls-Royce evolve the afterburning version of the Spey turbofan engine to fit the F-111K/M, probably rated as high as 23,000-24,000 lb. thrust, just below that of the later TF30-P-100 engine of the F-111F.
If I remember correctly, the F-111K was eventually developed into the FB-111A for SAC, essentially with longer span wings of the F-111B and the revised Triple Plow II intakes that cured the compressor stalls that plagued the F-111A. I think the F-111K would have used the Triple Plow II intake....