It is quite possible that the UK Government will extend this charge to EU workers who do not have rights of permanent residence once the UK leaves the EU. (lawyer-monthly.com)
British and other EU citizens who have lawfully resided in another Member State for five years have acquired a right of permanent residence, subject to conditions, in that State under EU law. (blog.politics.ox.ac.uk)
For those who have lived in their host state for more than five years, their right of permanent residence, which gives them a right to social assistance if necessary, will continue provided they do not spend more than five years continuously abroad. (europeanlawblog.eu)