I don't see it imposible for a civilization in Britannia to be more advance than in OTL, but I can hardly see it becoming as civilizated as Greece. Why? Because Greece was very close to the Middle East, one of the starting points of civilization and where many other advanced civilized states where. And as contact between Civilization make them become more advanced, because of the spread of knowledge, the Geographic situation of Britannia is going to make that contact and exchange more difficult.