There is a "massive morale bonus" - such as that units never ever rout if they defend a square. You have to kill them to the last man.
Also, british generals are incredible fragile. That always makes me wonder, because they should really have a load of HP.
Anyway, I only use my Generals to chase routers. Even there they are not efficient, because they just knock them down most of the time.
I only attack not routed units in very desperate situations. And then make sure to keep them moving. Charriots that stand still in melee with infantery are dead charriots. If your charriots ever seem to be stuck in a fight with inf, get them out.