If I recall right, the British had like 19 full squadrons of Spitfires at the outbreak of BoB. And they did a better job at keeping those numbers, than they did at keeping enough pilots.
So, it won't be perked for rarity or historical reasons.
That just leaves game balance, and I can see the point, who would pick a Hurricane, if the Spitfire was available everywhere?
For the same sake, who would pick a 110, if 109 is available everywhere....
Well, maybe it would be the best to limit the accessibility to fewer fields, yeah, I'd vote for that...