Is it unlawful for the UK to curtail the visas of Japanese nationals? Maybe. Maybe not. See Article 3(3) of the Treaty of Commerce, Establishment and Navigation between the United Kingdom of Great Britain and Northern Ireland and Japan (14 November 1962):
Any conditions as to the duration of his residence or as to his employment. profession, business or occupation which a national of one Contracting Party who is permitted to reside in any territory of the other is required to observe during the period of his residence in that territory shall he imposed at the time of the grant to him of permission to enter or to reside, shall be made known to him at the time when they are imposed and shall not thereafter be varied so as to make them more restrictive.
It wouldn’t just be curtailment that was against the treaty but also any downward variation of conditions of leave. But what is the status of the treaty and does it have any force?
Interesting spot by Tom Dyer, Managing Director at London Visa in Cyprus, thanks for sending it in!