If you hold a British Passport you need to apply for a visa before travelling to Tajikistan. You can check the entry requirements and any other relevant information on the FCO website. If you are planning to visit Gorno-Badakhshan Autonomous Region you will also need to apply for the GBAO permit. This can be done by stating that you need GBAO permit on your visa application; No separate application needed for GBAO permit.
My experience of applying for the Tajikistan visa at their London consulate was a very straight forward and painless process. To download the visa application form and review the requirements for the visa application you can visit the Visa Inquiries page of the embassy website.
Tajikistan Visa cost: £20
GBAO Permit cost: £50
Processing Time: 1 week