If you are fully vaccinated and 14 days have passed since your second vaccination, there will be no isolation or testing requirements if you are arriving from the Common Travel Area. This means you will be in the “blue channel” travel category. Accepted proof of vaccination status can be found here COVID Status Certificates (Vaccine Passports) | States of Guernsey - COVID-19 (gov.gg)
The Common Travel Area (CTA) includes the United Kingdom, the Bailiwick of Guernsey, the Bailiwick of Jersey, the Isle of Man and the Republic of Ireland.
Any fully vaccinated adult who had their vaccination administered in the Common Travel Area (Guernsey, Jersey, Isle of Man, UK or Republic of Ireland), EU or EFTA (Iceland, Norway, Liechtenstein and Switzerland), will be classified as Category 2 if they have travelled anywhere outside of the Common Travel Area in the previous 14 days – as long as they have not spent any time in a country on the UK’s ‘red list’.