Most brit tanks are lighter, correct? You have the same problem with the US Sherman tank:
if you add 1 light tank it totally sucks compared to the powerful heavy tanks in the game right now. You can't just add 1 light tank. You have to add a whole slew of light tanks (for other light tanks to fight), and in AH everybody's just going to take a panzr (or tigr, it seems) and be invulnerable to every other vehicle in the game.
I would like to see more tanks, lighter tanks, etc. I don't think they'll have a place in AH until the GV damage model is revamped to a point past its superficial self.