I can't remember what happens to single action air weapons over the poundage limit. Straight prohibited I think.
I know from the low tier (airsoft stuff) if it's over the power limit it becomes a high power air rifle (if it's single action) or a section 5 firearm if it's automatic.
Re the ball launcher thing, that's different as I believe there are exceptions for certain types of projectile (and more than likely a power limit though I'm not certain on that one).
The same applies to the flare gun, it doesn't have to be explosive powered to qualify as a firearm (see the previous Airsoft example). I think it's probably the fact it uses percussion caps (which would be ammunition) which require the license, which is why it's different to the others.
IANAL...
Biting the hand that feeds IT
