British passport holders arriving by air may enter Thailand for 30 days without obtaining a visa in advance of arrival. However, those arriving at overland crossings will only be given visa free entry valid for 15 days. If you plan to stay in Thailand for longer than the time endorsed on arrival, or you intend to work, you must obtain an extension of stay or a valid visa.
If you stay in Thailand for longer than the time authorised on your arrival, and you do not have an extension of stay or a valid visa, then this is an offence under Thai Immigration law. You will be fined 500 baht per day for every day you overstay, excluding the first day, up to a maximum of 20,000 baht. Any foreigner found by the authorities to have overstayed their visa is also at risk of being held in detention, fined and deported at their own expense. The Thai authorities have stated they will always enforce detention of overstays for more than 42 days. They may also be black-listed from re-entering Thailand. The only legal way of obtaining a new visa, entry permit or extension of stay is from a Thai Embassy or Consulate, an Immigration Officer at a point of entry into Thailand or one of the Immigration Offices around the country. Visas issued by visa shops, travel agents or by any other means are likely to be illegal and lead to criminal proceedings, which may result in a large fine or imprisonment. You would also be prohibited from entering Thailand again.