The Channel Islands are situated in the Bay of St. Malo off the Northwest coast of France and comprise of the Bailiwick of Guernsey (Guernsey, Alderney, Sark and Herm) and the Bailiwick of Jersey. They are self-governing Crown dependency with their own legislative assemblies, systems of local administration and laws and their own courts. Matters concerning defence and international relations are the responsibility of the British Government.
The Bailiwick of Guernsey is part of the joint customs union with the UK, Jersey and Isle of Man allowing tariff free movement of goods between the four territories of the customs union.
The Bailiwick will enter into a customs union covering all trade in goods involving the elimination of customs duty on imports and exports and any charges having equivalent effect, and will adopt a common customs tariff in relation to trade with any countries outside the customs union.
The text of the Customs Arrangement between the UK and the Bailiwick of Guernsey can be found here Guernsey-UK Customs Arrangement