Is roaming free in France with my UK or US SIM card?

It depends. France is in the EU, so "Roam Like at Home" applies only to EU citizens.

• If you are from the UK (Post-Brexit): Most UK carriers (like O2, Vodafone, Three) have reintroduced daily roaming fees (e.g., £2-£5 per day).
• If you are from the USA/Canada: Your home carrier (AT&T, Verizon, T-Mobile) will likely charge you a $10-$14 daily pass.

A Cellesim eSIM is a one-time prepaid fee, making it significantly cheaper than all these daily roaming charges.