https://www.nao.org.uk/wp-content/uploads/2017/07/The-Customs-Declaration-Service.pdf
https://www.gov.uk/government/news/pms-open-letter-to-eu-citizens-in-the-uk
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/640485/Technical_note_on_Functionality_and_Protocol_7.pdf
https://publications.parliament.uk/pa/cm201719/cmselect/cmsctech/705/70503.htm#_idTextAnchor001