You need to look at the actual science behind why plant based diets are better environmentally.
For starters, most soya is used in animal feed, followed by vegetable oil which is in basically every processed item of food, most of which are non vegan.
If we reduced the land used to farm animals and grow animal feed in the U.K. we could rewild it and support diversity.
Finally the vast majority of emissions are generated at the production, not the transport stage so the environmental impact of imported plants is lower than that of local animal products.