According to the most recent edition of Home Office Immigration Rules Part 8 Appendix FM-SE (20th November 2015):
"Where any specified documents provided are not in English or Welsh, the applicant must provide the original and a full translation that can be independently verified by the Entry Clearance Officer, Immigration Officer or the Secretary of State. The translation must be dated and include:
(i) confirmation that it is an accurate translation of the original document;
(ii) the full name and original signature of the translator or an authorised official of the translation company;
(iii) the translator or translation company’s contact details; and
(iv) if the applicant is applying for leave to remain or indefinite leave to remain, certification by a qualified translator and details of the translator or translation company’s credentials."
I had our marriage certificate translated by a company at Mahatun Plaza in Ploenchit where I was studying Thai. I asked them to have their translation certified by the Thai MFA. I then had this MFA certified copy further endorsed with a Certificate of Legalisation at the British Embassy to make future applications for a Schengen Visa easier as I've seen that some EU member states can be a trifle awkward about non-EU marriage documents when it comes to issuing these visas to non-EU spouses.