Hmmm .... so would Unite also be arguing then that UK based employees of foreign airlines, and indeed ALL foreign companies, should be covered by the agreements, T&Cs and laws applicable in each carriers/companies homeland? I would certainly hope not!
Back in the 80's, I worked here in the UK for a foreign state airline. The Government owners of that airline, involved in a lengthy civil war, imposed a 10% War Levy on all employees in the home nation, and tried to also impose it on all employees of the airline, worldwide. It was very quickly pointed-out to them that UK employees worked under UK agreements, T&Cs and laws.