I was raised in England in the 60s and 70s and vegetables were indeed generally cooked too much. However, I think things have changed. As an adult I usually had steamed, slightly crisp vegetables. On moving to the states, my tastes proved to be for firmer veg than the locals in general. That could be regional, though - Southern food can certainly cook things to a mush at times.
As to why vegetables were overcooked - no idea.