wuzak
Captain
In the case of the Defiant, it became an effective night fighter. The Botha was a disaster. The Lysander could still have been useful in the event of an invasion, and used like the Po-2 in the USSR as a light night bomber. By the time the Spitfire III comes along, we're struggling to get Spitfire II production up and running. We need to keep the Hurricane competitive. The Hurricane is our most numerous front line fighter, its cheaper to build than the Spitfire, and while it is taking heavier losses, in terms of cost, it is still competitive in unit costs to the Spitfire. So if the Spitfire destroys 1.7 enemy per loss and the Hurricane only 1.1, but the Hurricane is 2/3 the Spitfire's unit cost and is easier to train to use and more rugged and usable overseas then it is the better fighter so it deserves the better engine.
Aircraft can be replace, pilots are more difficult, especially experience pilots.
Losing 50% greater numbers of Hurricanes than Spitfires may be more cost effective, but when you include the value of the pilot, maybe not so much.