I am travelling on a british passport.Do I need to get a visa to enter Thailand?

You do not need a visa to enter Thailand, if your stay shall not extend 30 days.

If you would like to extend your stay after 30 days, you may do so by visiting the local immigration office and apply for an extension of 15 days.

After you have filled in the extension application and you surrender currently (24.12.09) 1,900.- Thai Baht to the immigration office they will stamp for you the 15 days extension.
Everything can be done at spot and is straight forward. You need your passport and two pictures. Even this can be done at the Immigration Office.

For longer stays you may consult your embassy or consulate in your home country.

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