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

https://ukandeu.ac.uk/long-read/the-protocol-post-brexit-northern-ireland/?mc_cid=9de7c359e6&mc_eid=815b1928ec

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/665871/December_-_Joint_technical_note_on_the_comparison_of_EU-UK_positions_on_citizens__rights.pdf