If residents of the Foggy Albion permanently reside outside the United Kingdom but want to marry each other, they must first take care to obtain a Certificate of No Impediment. This document allows you to recognize the marriage concluded outside of England between British citizens as legal.
You can obtain such a document at any British consulate. Usually it takes from 15 to 21 days to issue this document. The document you receive should be apostilled and then notarized. Also the document should be translated into the national language of the country where the marriage is planned.