Matt, thanks for input but just two things : you have stated twice that the UK "does not recognise same sex marriage" - it absolutely does - and irrespective of the legislative "level" at which marriage takes place (the honest answer is I don't know) the current issue is that his Visitor Visa specificially precludes it as well as work and study

The suggestion of using a third country to get married is one I will certainly look into.

The issue is going to be whether my BF can obtain a Visa/Schengen Visa for that "third" country (notoriously difficult for Vietnamese to gain lawful entry to anywhere outside of the Asean countries) and whether the conditions of that Visa and the laws of the particular country allow or preclude entering into either a CP or marriage.

A preliminary search suggests that Latvia and the Czech Republic are the easiest Schengen countries to obtain a visa from (99% approval apparently) - but I've yet to search their same-sex policies