http://www.cer.eu/sites/default/files/pb_ireland_brexit_6.7.17.pdf
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/680366/Technical_Note_-_International_Agreements_in_the_Implementation_Period_-_CLEAN.pdf
https://www.nao.org.uk/wp-content/uploads/2017/07/The-Customs-Declaration-Service.pdf