Do British Citizens need a visa for Thailand?

No, British passport holders are granted a visa exemption and can stay in Thailand for up to 30 days without a visa. You must hold a valid British passport with 6 months minimum left from the expiry date.

In order to work or study in Thailand you will need to apply for a visa from the Royal Thai Embassy.

Do they ask about criminal records?

No. There is no reference to criminal records on the Thai T.M.6 Arrival Card (see below) or on the Visa Application Form.