So, quite a few months after the end of ARC's What If GB I've finally got one of my entries done!
I've settled for Westland because, innovative though Martin-Baker were, I think a more established company would have been selected to develop jet fighters in my slightly earlier timeline than reality. Westland could have been ordered to stop work on developing the Welkin, but the Whirlwind would still have been produced in limited numbers to give the RAF experience with 2-engined fighters. As Westland had proposed a jet development of the Whirlwind in reality, maybe Westland could have