Citizens of United Kingdom can enter Thailand visa free for up to 30 Days. You only need to purchase a visa if you intend to travel to Thailand for longer than 30 Days.
If you are a UK Citizen entering Thailand for 30 days or less for tourism purposes, you will qualify for the visa exemption scheme. However, you are still required to provide the following documentation at the point of entry into Thailand:
Proof of adequate finances for the duration of stay in Thailand i.e. traveller’s cheque or cash equivalent to 20,000 Baht per person and 40,000 Baht per family.
Proof of onward travel (confirmed air, train, bus or boat tickets) to leave Thailand within 30 days of the arrival date.
Your passport should be valid for at least 6 months from the date of entry.