FWIWAn interesting possibility might have been either BAC or Hawker Siddeley building the Mirage III under licence for the RAF. The UK would have had some bragging rights over France as the Fairey Delta was tested at the Cazaux site and Dassault used data from the Delta to develop the Mirage III. A favourable deal could have been negotiated.
The most likely power plant would have been the Rolls-Royce Avon or Spey. I understand that Australia used the Avon in the Mirages they built for the RAAF under licence.
- There was a proposal for a Spey powered Mirage III IOTL. It was called the Mirage IIIK.
- One Mirage III was fitted with an Avon and it first flew in that from on 13th February 1961. It was evaluated by the RAAF who decided that the improvement in performance wasn't worth the increase in cost. So the 116 Mirages built for the RAAF had Atar engines. That's according to "Modern Combat Aircraft 23 Mirage" by Paul Jackson.