https://blogs.sussex.ac.uk/uktpo/2017/06/12/disentangling-brexit-trade-politics-and-policies/
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/640485/Technical_note_on_Functionality_and_Protocol_7.pdf
https://blogs.lse.ac.uk/brexit/2021/02/04/indyref2-scottish-independence-would-be-2-3-times-more-costly-than-that-of-brexit-and-rejoining-the-eu-wouldnt-make-up-the-difference/
http://openeurope.org.uk/today/blog/will-brexit-ground-flights-europe/