Can I enter Thailand visa-free and extend my stay on the same day if I arrive in Bangkok from Vietnam?

You can enter Thailand visa-free as a British passport holder and are eligible for a 30-day visa exemption upon arrival. However, you cannot purchase a visa extension at the airport; the extension needs to be done at a local immigration office in Thailand. Extensions can be applied for up to 30 days at your local immigration office, and it’s generally advised to apply for the extension a few days before your current exemption expires.