A mixed forest with oak, beech and the like, as you can find higher north. But that was during the middle ages' European "little ice age", when the winters were much colder, and the Thames and the Netherlands froze over. That's also likely the reason why the forest was cut so much to feed fires.
No, it shouldn't. It used to be a forest but was cut down by firewood and sheep grazing.
Stop with the "return to nature" fallacy. With how things are going, even "natural" forests are going to burn down.