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

It is complicated.

• For EU Citizens: Yes, "Roam Like at Home" (RLAH) technically applies. However, many non-French carriers (e.g., from Germany, Spain) enforce strict "Fair Use Policies" for remote territories and may cap or throttle your data.
• For UK/US Citizens: No! Post-Brexit UK plans and all US plans will charge you very high international roaming fees ($10+/day).

A Cellesim eSIM guarantees a fixed price and high-speed data without "Fair Use" limits.