https://www.nao.org.uk/wp-content/uploads/2018/10/The-UK-border-preparedness-for-EU-exit.pdf
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/704057/Policy_Note_-_Exceptions_and_Licences.pdf
http://www.resolutionfoundation.org/app/uploads/2017/10/Changing-Lanes-3.pdf
https://www.instituteforgovernment.org.uk/sites/default/files/publications/IfG_Brexit_Euro_Court_Justice_WEB.pdf