Is roaming free in Thailand if I'm using my UK or US SIM card?

It's highly unlikely that roaming would be free in Thailand with a standard UK or US SIM card. Thailand is not part of any 'free roaming' agreement for most European or North American carriers. Post-Brexit, many UK carriers (like Vodafone or Three) have reintroduced daily roaming fees for non-EU countries, which can be quite expensive. Similarly, US carriers often charge significant daily passes or per-MB rates for international roaming. An EastESIM plan for Thailand, connecting you directly to local networks like AIS or TrueMove H, offers a far more predictable and cost-effective way to stay connected without bill shock.