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

No! This is the #1 roaming trap. Although Jersey 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 (like O2, Vodafone, EE) DO NOT APPLY. Most will charge you expensive international roaming fees (e.g., £2-£6 per day). A Cellesim eSIM is a one-time prepaid fee that is significantly cheaper.