Is roaming free in the UK with my European (EU) SIM card (Post-Brexit)?

No! This is the #1 roaming trap. Since Brexit, the UK is NOT part of the EU's "Roam Like at Home" (RLAH) policy. Most EU carriers (like Orange FR, T-Mobile DE, Movistar ES) will now charge you expensive international roaming fees. A Cellesim eSIM is the only way to get a fixed, cheap data price and avoid a massive shock bill.