It is important to understand that Guernsey, Jersey, and the Isle of Man are not part of the United Kingdom. They are Crown Dependencies and each has their own independent government.
Nor are they part of the European Union (EU) or European Economic Area (EEA), and so are not subject to UK, EU, or EEA banking regulations or supervision; each jurisdiction regulates and supervises itself.
This means that if your bank gets into difficulties you will be reliant on only the local jurisdiction for help, and not any other country. The UK has explicit stated that they will not assist you or them