Thank you all so much, these replies have been so helpful. I think I'm going to focus specifically on the Defiant and try to examine in more detail why it was used in a role it was so clearly unsuited to.
Bear in mind the circumstances, as previously pointed out. When Churchill is giving orders for training aircraft which includes Tiger Moths to be ready for the call of battle by dropping hand release small bombs if an invasion happens, as Gingerbob posts above, the posting of Defiants to a front line area becomes more understandable.
Not until after June 24th 1940 were they fitted with props being able to be feathered.
Initially Merlins were fitted with the wooden 2 bladed Rotol propeller which had a 2 pitch setting, fine pitch for take off and coarse pitch for all in flight operation, meaning it was a buggers muddle of too much on take off and not enough in flight with the engine not being able to give as much power as it was capable of.
De Havilland having a strong civilian aeroplane background had developed in house constant pitch propellers which were able to be feathered for some of their civilian offerings, because they were heavier and offered the possibility of going wrong not fitted to military.
A hasty call to De Havilands from the official at Beaverbrooks ministry supply department in early June 1940, of the sort such as "now look here, we understand you chappies have some of these contracting propellers" (noises of...no sir...constant pitch...constant pitch, heard in the background)" hrrumph yes that's what I said, damn your eyes Perkins...yes constant pitch. Well top priority old boy can you have a look at developing some of these thing's (noises of...how many do we need Perkins 2 or 3...WHAT 300 for conversions ! well it's your pension !) and a constant supply to match all production of Merlin engine units ?"...
"Hhhmmm" De Havilland man bites hard onto his pipe stem, "well we need to develop a hydraulic locking unit with variable capacity to take the Merlin and it's power, lots more than any commercial application don'tcha know ? ( puts feet up on his desk crossing his dark brown brogues) and of course an external hydraulic feed with integrated linkage to in cockpit controls and feathering capacity, which models need converting first ? "
"Spitfire, Hurricane and then a couple of squadrons of Defiants, can we pencil you in for the 10th June to commence"...
"Cough errr we may need a little more time, how about I send a telegram 24 hours before commencement ?" ( heard in background OOOhhh sir...sir ask him for an order), Oh... damn paperwork, I don't suppose you could forward an order could you ?"
"Bit pressed at the moment old boy, don't worry we aren't going anywhere, ha ha, we'll catch up with it later, action this day as the great man says what ?
"Fine says De Havillands best, make it so Mr Jenkins, make it so." ("bugger...thinks the guy from accounts, I bet we never get paid for these")".
So 4 days later 2 sweating and greasy (the hydraulic pipe blew off twice) engineers demonstrated the modified and now signed off constant pitch propeller on a Spitfire loaned by the RAF to the pleased De Havilland directors, and a telegram was sent arranging fitment and proposed dates 8 days away to Beaverbrooks office . A convoy of trucks and engineers from DH left on the 24th June to the various aerodromes and the fitments then completed within a week, allowed the RAFs Spitfires and Hurricanes to gain 7000 feet in height ceiling and 20 seconds reduced climb to 15,000 feet (absolutely essential when climbing to attack levels within a tight time frame) and the pilots benefited from an easier and more flexible plane to fly with the benefit now of having a propeller capable of being feathered when occasion demanded.
Now I've dramatized the above speech and thought processes but something like that happened and that's how the Merlin got it's constant pitch propeller capable of being feathered. Oh...except truthfully,...the guy from accounts was right, the order from Beaverbrooks department never followed on, so De Havilland carried out all the work on trust. They had a battle from Beaverbrooks office because Beaverbrook had been moved on and DH didn't have any paperwork to justify the extensive conversion costs and onward supply costs for the Merlin contracting propellers, "dammit Perkins, I know what I mean ! "
The two Defiant squadrons had received the updated De Haviland constant pitch propellers which were fitted to Spitfire and Hurricane squadrons first as a priority, in June 1940, and when measured up against Gladiators and the other training and "next best"aircraft available were the closest we had to front line fighters after Spitfire and Hurricanes.
Most tactically aware air force commanders knew that their aircrafts life would likely be short but after Dunkirk and under the premise that "if what we have isn't a mess, it will do until a real one comes along ! belts were being notched up and "game faces" were being put on.