https://www.nao.org.uk/wp-content/uploads/2018/10/The-UK-border-preparedness-for-EU-exit.pdf
https://ec.europa.eu/commission/commissioners/2014-2019/andriukaitis/blog/likening-eu-ussr-not-unreasonable_en
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/677268/Letter_to_Business_Leaders__1_.pdf
https://publications.parliament.uk/pa/cm201719/cmselect/cmpubacc/401/401.pdf