Is roaming free in Serbia with my European (EU) or UK SIM card?

No! This is the most common mistake. Although Serbia is in Europe, it is NOT a member of the European Union (EU). Therefore, the "Roam Like at Home" policy does not apply. Most EU and UK carriers (like Vodafone, O2, T-Mobile) 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.