All UK citizens require a visa to enter Thailand, irrrespective of the length of stay. ('Visa on arrival' facilities, for stays of up to 15 days, are available for visitors from 20 nominated countries but these do not include the UK).
The validity of a tourist visa, obtained in advance, depends upon the nationality of the visitor. It will be either 30 days or 60 days. For UK passport holders, the validity is 60 days.
Source:
http://www.thaiembassyuk.org.uk/visa_tourist.h tml
Visa application forms can be downloaded from here:
http://www.thaiembassyuk.org.uk/visa_dl.html
Postal applications are not accepted. Applicants should attend the Thai Embassy in London, between 0930 and 1230. The fee is �25. (Some travel agencies or visa agents might also be able to accept the application but they will probably charge an additional fee):
http://www.thaiembassyuk.org.uk/visa_inst.html
Chris