TOURIST VISA FOR UK PASSPORT HOLDERS OR UK TRAVEL DOCUMENTS
If you are a British Citizen you can obtain tourist visa either at the port of entry in Turkey for £10.00 or from this Consulate General for £ 68.00.
The sticker visa obtained at the port of entry in Turkey will grant you 90 days stay with multiple entry to Turkey starting from the date of issue. (eg: the sticker visa issued on 5 May 2010 expires on 2 August 2010).
British National Overseas, British Subject and British Protected Person passport holders are not issued sticker type of visa at the port of entry in Turkey. They should obtain their visas from this Consulate General before traveling.
If you are not a British passport holder; yet you have a travel document (not Certificate of Identity) issued by the Home Office of the UK, you need to obtain visa before traveling to Turkey. For more information on what a travel document issued by the UK is and upon what basis it can be issued, please see "http://www.homeoffice.gov.uk"
Please note: tourist visas do not give you the right to take up paid or unpaid employment or to reside, or to study (including student exchange program) or to establish yourself in business in Turkey.
Taken from here
http://www.turkishconsulate.org.uk/en/visa.asp?PageID=2#2