The answer is yes, you can go back to Norway (starting area) at any time in Assassin’s Creed Valhalla.
Once you reach England, open the World Map and press Triangle (PlayStation) / Y (Xbox). It will say “Atlas” and the button on the World Map (bottom left corner).
Then select Norway from the Atlas and you will fast travel there instantly.
It works the same way with Vinland, another area unlocked later while going for all Order of Ancients Members.
So yes, you can return to Norway at any time and fast travel between Norway and England at any point. Even after the story you can still free roam and return to all areas.
For more guides check out the complete Assassin’s Creed Valhalla Wiki & Strategy Guide.
But are Quests available when you arrive back in Norway. I see only England Quests now. I want to know what I missed