The problem with this was that using heavy AA in an AT role is one of those things that can only really come about by accident. If you, knowing what you know, were allowed to decide which gun to use, of course you'd plump for a heavy model. People in the late 1930's didn't have this advantage and instead just decided that smaller calibres were quite sufficient for the tasks they envisaged their armoured vehicles performing.
Case in point: Britain had a quick-firing 3.7 Inch (i.e., 94 millimetre) anti-aircraft gun. As an anti-tank gun it might have been outstanding, though it was a lot heavier and harder to manoevre than the German 88 millimetre gun. If the player is simply allowed to say "I'll have the 94 mm gun please" at an early stage of course they'd do it every time. Instead, whilst it should be possible to equip units with such a heavy gun, it should be expensive to develop and difficult to manufacture.
Indeed but it´s a matter of balancing the game, instead of removing the possibility. HOI 3 was full of silly units values. Tank destroyers being so good on the attack one of the prime examples.
I fail to see why the brits can´t have dual purpose AT guns. Just make it expensive and clumsy on the attack, and it´s done. Remember the only situation UK was on the defensive was in North África and France. What are you going to do with all those heavy guns later?
Also, 88 was the basis for the first really good anti-tank gun placed on a tank. Why the brits should have access to a better armed heavy tank if they invested in a good AT gun, as well? The first decent brit AT gun was the 17 pounder, which arrived maaaaany years later than the 88.