The Hague Convention, abolishing the requirement for Legalisation of Foreign Public Documents (“Hague Legalization Convention”) has been in force in Bulgaria since 2001.
If you need to use official British documents in Bulgaria, be it for business purposes (registration of companies, trademarks, and patents) or personal and family reasons such as getting married in Bulgaria, they have to be legalized at the Legalization Office of the Foreign and Commonwealth Office in London.
Internet site: http://www.fco.gov.uk/legalisation