The Channel Islands are not 'in' the EU Single Market, but have a close relationship with it; this is particularly true in Financial Services, where they have a wide-ranging Equivalence agreement. Post-Brexit, the UK has been refused Equivalence under the current system, and has to rely on the 'Temporary Permissions Regime' instead to serve EU27-based clients. For activities that require Equivalence, but are not covered by the Temporary Permissions regime, could the UK use the Channel Islands as a base for this?
It is not clear from paragraph 36 in the following reference whether the above-mentioned scenario is covered:
While on the other hand the reference below insists that the Channel Islands relationship with the EU will be practically unchanged: