
https://www.publications.parliament.uk/pa/cm201617/cmselect/cmintrade/817/817.pdf

https://www.nao.org.uk/wp-content/uploads/2017/07/The-Customs-Declaration-Service.pdf

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/725288/The_future_relationship_between_the_United_Kingdom_and_the_European_Union.pdf