Is roaming free in Guernsey with my UK or EU SIM card?

No! This is the most common mistake. Although Guernsey is a British Crown Dependency, it is NOT part of the UK and NOT in the European Union. Therefore, "Roam Like at Home" (EU) and standard UK carrier plans DO NOT APPLY. Most UK and EU providers will charge you expensive international roaming rates (e.D., £2-£6 per day or £2 per MB). A Cellesim Guernsey eSIM is far cheaper and saves you from a massive shock bill.