https://www.nao.org.uk/wp-content/uploads/2017/07/The-Customs-Declaration-Service.pdf
http://openeurope.org.uk/today/blog/will-brexit-ground-flights-europe/
http://www.transfrontier.eu/regions/ireland-northern-ireland/
http://www.epc.eu/documents/uploads/pub_7576_northernirelandandbrexit.pdf