Legal framework for exchange: Agreement versus Arrangement
The UK’s Crown Dependencies (the Isle of Man, Guernsey, and Jersey) and Overseas Territories (for example the British Virgin Isles, the Cayman Isles, and Gibraltar) are part of the sovereign state of the United Kingdom. Hence, under international law principles, there cannot be an international agreement between the metropolitan UK and these jurisdictions, or between the jurisdictions themselves, because a state cannot treat with itself.
The Crown Dependencies and most of the Overseas Territories have their own tax or finance authorities; HMRC cannot directly access information held by these agencies, and nor do they have automatic access to our information. Instead, the UK will make ‘arrangements’ to exchange information. These will normally be in the form of Double Taxation Arrangements or Tax Information Exchange Arrangements.
These ‘arrangements’ are functionally the same as their equivalent ‘agreements’, with the same principles of foreseeable relevance and reliance on Competent Authorities. They are based on the same models the UK would use to negotiate an international Agreement. In some cases, they may be entitled ‘agreements’ and use binding treaty language, but in international law they are still ‘arrangements’ and not international agreements.
In practice, you can consider an ‘arrangement’ with one of the UK’s territories or dependencies to be the same as any other exchange instrument.