
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/650455/customs_bill_white_paper_print.pdf

https://blogs.lse.ac.uk/brexit/2019/04/15/brexit-lessons-from-the-silesian-backstop-of-1919-25/

https://www.instituteforgovernment.org.uk/sites/default/files/publications/IfG_Brexit_customs_WEB.pdf