The decision was political as much as anything else.
By the time these decisions were made Dowding and Park were out, Douglas and Leigh Mallory were in ... and they were a bit duff (well actually very duff).
They way their minds worked was like this:
No Spits anywhere else except the UK, which is why it took so long for Malta and North Africa to get them (and they fought to the bitter end to stop any going there).
Took until March 42 for Malta and the end of 42(!!!) for NA to get Spits there, despite the slaughtering of the Hurricanes and P-40s by the 109Fs and later 109Gs.
'Leaning forward to the enemy'. One of the most useless air tactical plans ever made (comparable to the Battle of Berlin), where they sacrificed vast quantities of pilots trying to gain supremacy over the Luftwaffe in France.
That was the main reason for their push for the Spit Mk V, to get something equal to the 109F quickly so they could send them over to France and engage.
They were impatient and really didn't have a clue what they were doing (and never seemed to learn).
But since the Hurri's were for the 'minor areas', it seemed like a good idea to give it a bit more poke to try and remain competitive (it didn't work), so they wasted the Merlin XXs.
In the end they got the worst of all possible worlds. Slaughtering in Malta and North Africa of the 2nd rate fighters there.
Slaughtering of Spit pilots in France (because the tactical plan was totally flawed).
Between 41 and 42 they were probably responsible for the waste of at least 1,500+ pilots and achieved very little for it (in France they achieved nothing).
The logical thing to do (and if Dowding and Park were still there it would have probably happened, since they were very logical people) was:
Spit III into production (LF and HF versions), upgraded engines over time for better prformance (ie 400mph), the original engine was the Merlin X, which I think those performance figures come from.
Spit IIs sent to Malta and North Africa in solid numbers in 41. Any Hurris and P-40s purely for ground attack.
I'd still make the Spit Mk Vs (in far lower quantities) for the overseas forces only, but still replace as sufficient IIIs came on-line.,
Maintain sufficient for home defence, but after June 1941 only a nominal force was required, so there was time to build up the MK IIIs.
Naturally bring in Merlin 60 series Spits as they became available.
Come up with a tactical plan to wrest air supremacy over France and the lowlands (yes it was necessary).
Make (or modify) the equipment to achieve it (which they didn't have), which meant longer ranged Spits and Mosquitos (and P-51As as they came on line), to fight the Luftwaffe in the air, chase them to their airfields and hammer them on the ground.
Create the force structure necessary, test of course with various trial operations. That would have taken 41. Then hammer the Luftwaffe in 42.
Results?
Malta: they would always have had air supremacy over it and that long bombing siege would never have happened.
Air supremacy in North Africa right from the beginning and would never have lost it.
Air supremacy over France (and the lowlands) by the end of 42, setting the scene for the 43 bombing operations of the US.
Far, far fewer pilots wasted needlessly.
I should add that the DAF was very lucky in North Africa that the opposing Luftwaffe fighters were idiots. If they had done their job properly they would have probably eliminated the DAF as an attacking force totally.