Is the United Kingdoms population too high?
It’s too high, but the only way to convince people to go elsewhere is to make elsewhere more prosperous. If there are a lot of jobs in the Sudan, for example, people from Sudan and the surrounding regions will go there. The other key is to promote stable governments and trade. Recently they started talking about building a hugh solar energy array in the Sahara Desert. That will provide some short term jobs, but with a little thought, it would provide a lot of permanent jobs. If they add a desalination plant and a pipeline to provide water to the area and build the array 12 feet off the ground so housing, water lines, and gardens could be added underneath it, you could make the Sahara bloom AND become a new nation.