http://fedtrust.co.uk/views-from-the-federal-trust/free-movement-and-migration/
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/643924/Foreign_policy__defence_and_development_paper.pdf
https://www.bloomberg.com/news/articles/2019-09-11/world-s-oldest-customs-union-and-u-k-reach-a-post-brexit-deal