In his recent decision in the case of The States of Guernsey & Anor v Secretary of State for Environment, Food and Rural Affairs & Anor  EWHC 1847 (Admin) (22 July 2016), Mr. Justice Jay of the English High Court’s Administrative Court had to consider an unusual case in which the Applicant was the local Government of Guernsey, which sought judicial review in the English Courts of the UK Government’s decision, through DEFRA, to indefinitely suspend the Fisheries Management Agreement between DEFRA and Guernsey, and associated reciprocal fishing rights.
Leaving aside the fascinating discussion in the judgment relating to fisheries policies and laws, and the justiciability of executive actions under the Prerogative, there is a short and incomplete summary of Guernsey’s relationship with the UK, as follows:
“The island of Guernsey is not part of the UK (see, for a detailed exposition of the nature of the constitutional arrangements between Guernsey and the UK, paragraph 6ff of the judgment of Baroness Hale DPSC in R (oao Barclay and another) v SSJ and another  AC 276). Guernsey, together with Jersey and the Isle of Man, is a Crown Dependency. In theory, UK Government and Parliament have full sovereign powers in relation to Guernsey, but in practice the relationship is more collegiate and facilitative, guided by a number of constitutional Conventions designed to accord Crown Dependencies as much autonomy as possible, in the context of their own democratically elected legislatures and municipal organs of State. As paragraph 13.4 of Defra’s Detailed Grounds of Defence explains, “the UK does not legislate or intervene in the domestic affairs of the Crown Dependencies without their consent, other than in very limited circumstances”. It is convenient to leave such circumstances undefined.”
The final sentence, “it is convenient to leave such circumstances undefined“, rather begs the question as to what sort of circumstances might justify the UK Government’s active intervention in the domestic affairs of the Crown Dependencies (and also the British Overseas Territories). There must be some, albeit “limited”.
See also, for a fuller discussion by the UK Supreme Court on the same topic: Barclay & Anor, R (on the application of) v Secretary of State for Justice & Ors  UKSC 54 (22 October 2014)