https://www.publications.parliament.uk/pa/ld201617/ldselect/ldeucom/169/169.pdf
https://committees.parliament.uk/oralevidence/2766/pdf/
https://www.socialeurope.eu/2017/06/britains-deepening-confusion/
https://www.nao.org.uk/wp-content/uploads/2017/07/The-Customs-Declaration-Service.pdf