On this page of the Assassin's Creed Valhalla Siege of Paris guide you will find out how to get back to England.
You can return to England in two ways. The first way does not make the gameplay any less immersive. You can select the option "I want to go back to England" when completing the mission at Pierre's. This will bring you back to the main game map.
However, if you care more about time than immersion, go for the tried and tested method. Open the world map, then go to atlas (triangle on PS4/PS5). This way, you can get to:
- Ireland (if you have the Wrath of the Druids DLC),