The recognition in the UK of a marriage which has taken place overseas can be determined only by a court. I understand, however, that generally such marriages would be recognised here as valid provided that they complied with the law of the country where they took place and the couple had the legal capacity to marry each other.
There is currently no provision in law for marriages which take place outside the UK to be registered in this country. But, under the Foreign Marriage Order 1970, certificates of marriage may be lodged with the Registrar General on receipt from the local consular office of non-Commonwealth countries. However, if the couple are already validly married to each other, there would be no need for the marriage to be registered again in this country.