A good foundation, but now it needs building on.
Hurricanes may not be as fast as a Spit, but they were available in more numbers and were (at the time) a better gun platform. They accounted for over 60% of all air kills in the BoB. I would suggest strengthening the Hurricane.
Spies need to be stronger and more survivable. British Intelligence was far stronger than German Intelligence in the war (all German agents were double agents controlled by the British).
I would add AA batteries. Uses manpower, extremely low chance of a kill, but cannot be shot down (obviously).
Check the combat script. My example of "yeah, right" was 2 Stukas nobbling a pair of Spits. It could happen, but even a Defiant was more than a match for a Stuka.
The only thing that annoys me about this game is the very small radius of money collection even after upgrading. This is the main thing that held me back from the "Mad Scientist" achievement - lack of cash. Having a radius of 200 pixels would probably have increased the cash flow by about 50%, and had me hovering just off the edge, instead of being miles off (depending on how many total upgrades there are).
As for the Heros - Rambo, Sanders, MJ and Ronald certainly appear in multiple locations. Spidey might do as well, but sometimes they didn't live long enough for me to see who had walked on the screen and been executed.
No, falkenmond, the square root symbol is quite different - there's a little "v" just in front of it.
That's one of the symbols used for longhand division (and a related sum type double division).
 "Spitfire: 1940"){kind=icon}
 "Neon Race"){kind=icon}
 "The Final Death Wish"){kind=icon}
 "Infectonator : World Dominator"){kind=icon}
 "Dinx-School Spirit"){kind=icon}