https://eupinions.eu/index.php?id=12463
https://www.nao.org.uk/wp-content/uploads/2017/07/The-Customs-Declaration-Service.pdf
https://blogs.sussex.ac.uk/uktpo/2017/06/12/disentangling-brexit-trade-politics-and-policies/
http://blogs.sussex.ac.uk/uktpo/files/2017/11/Briefing-Paper-11.pdf