The action is set on Orkney and in the view from the plane forests can be seen but there are no trees on Orkney, or just a few very small ones. The climate can't support large trees.
The secret base is in the Orkney Islands which appear well-wooded from both the aerial shots and characters fleeing through the woods. Orkney is practically treeless.
The fleeing scientist at the start runs into what is supposed to be a Scottish village, but the architecture of the buildings is very typically English.