British citizens arriving by air may enter Thailand for up to 30 days without obtaining a visa in advance of arrival. Nevertheless, those arriving at overland crossings will only be given visa free entry valid within 15 days. If you intend to stay in Thailand for longer, or you intend to work in the country, you must get an extension of stay or a valid visa.
It is prohibited to stay longer without the proper authority. Failure this you will be arrested until a fine is paid. To get a new visa, entry permit or extension of stay legally you have to apply to the Royal Thai Embassy or Consulate, an Immigration Officer at a point of entry into Thailand or one of the Immigration Offices around the country.
Keep in mind that visas issued by visa shops, travel agents or by any other methods are probably to be illegal and lead to criminal proceedings, which may result in a large fine or imprisonment. In addition, you would also be prohibited from entering Thailand again.
You can’t enter to Thailand if you have a passport with less than six months’ validity, or damaged, or if pages are missing from it.
If you want to work here, you need a work permit, which is difficult to obtain and time consuming, in order to work legally in Thailand. If you enter Thailand on a tourist visa you are not allowed to take up employment. Failure to observe this rule can lead to arrest and deportation. | 
