That's my point. The guys making this game obviously know rather a lot about aircraft, and even if they didn't, a 2 minute search on wikipedia would tell them about even the most obscure aircraft variants.
These models and textures take a hell of a lot of time and money to create. They obviously choose very carefully exactly what kind of aircraft they want to make a model of. And for whatever reason, they chose these ones.
Remember, singleplayer isn't their only consideration. They probably want to have the Spitfires and Hurricanes balanced nicely with the Bf 109 for multiplayer, and if the Bf 109 has a cannon armament and the British planes don't, that could be a big problem.
Either way, remember that this is a flight sim, not a time machine. Shooting down He 111s over the channel isn't going to feel quite right, because you're sitting in a couch looking at at TV screen rather than flying the real thing.
