Same sex marriage in the UK for foreigners on vacation

I believe you should be able to form a civil partnership or marriage in the UK as foreigners, regardless of it being same-sex or not.

From 29 March 2014, same sex couples can get married in England and Wales. You can only get a civil partnership as a same sex couple.

There’re some differences being made between marriage and civil partnership, but as far as they’re concerning intra-UK regulations they should not concern you. Acceptance of civil partnerships in other countries with the same status as a marriage may or may not be the case and depends on the respective country’s laws – more on this towards the end of this answer. A marriage should be more widely accepted, but I’ve not come across any information on that.

Information on going to the UK to get married is found on the Marriage Visitor Visa page of the UK government. Also their information page on Marriage and Civil Partnerships says the following:

Foreign nationals

You and your partner must go to a ‘designated’ register office if
either of you aren’t from the European Economic Area (EEA) or
Switzerland, and are subject to immigration control.

The process is different in Scotland and Northern Ireland.

You’ll need to apply for one of the following if you aren’t already in
the UK:

• a visa to come here as a fiancé(e) or proposed civil partner if your partner is a British citizen or settled in the UK and you intend
  to remain in the UK
• an EEA family permit if you’re from outside the EEA and the person you’re marrying is from the EEA or Switzerland but not the UK
• a Marriage Visitor visa if you and your partner are from outside the EEA or Switzerland, you want to come to the UK to get married or
  register a civil partnership, and you and your partner intend to leave
  the country within 6 months

You must get married or register your civil partnership within 1 year,
or 3 months if you’re in Scotland.

The third bullet point there should be your case, but do visit the two linked pages and find all the detailed information there.

Recognition abroad:
Recognition of UK civil partnerships in other countries depends on the local laws and no general statement can be made. Overall though it seems in Europe a civil partnership would be accepted as a marriage or (if existing) something equivalent to a civil marriage in most cases – see here for some info. In the US this seems to depend on state law – again the mentioned site has some info.

The following is a list of countries that accept UK civil partnerships in some form or the other – anyone finding more info, feel free to add to the list:
(statements enclosed in “” are embassy statements according to the sources)

Europe

• Austria: “Austrian Law offers a total legal equality for your official civil partnership (same sex marriage).” src
• Belgium: “Until legal proof of the contrary, a British civil partnership is regarded as the equivalent of the Belgian civil union/marriage” src
• Czech Republic: “In the Czech Republic exists “registrated partnership”, which is a recognition of same-sex couples relationships. Foreign equivalents of registrated partnership are recognized.” src
• Denmark: “If you have a Civil Partnership Certificate your partnership will be recognized in Denmark.” src
• Finland: “Thank you for contacting the Embassy of Finland. Your British civil partnership will be valid in Finland; you will, however, need to be able to produce the original, signed and stamped copy of your civil partnership certificate when you register at your relevant local Register Office (Maistraatti).” src
• Frace recognises the British CP and gives them tax rights and social rights – not necessarily the same rights as PACSed couples src
• Germany “British civil partnerships are recognized by German authorities and institutions as an equivalent of the German ‘Eingetragene Lebenspartnerschaft’.” src
• Hungary “Civil partnerships contracted according to UK law are recognised in Hungary on the same basis as the registered life partnership recently introduced in Hungary.” src
• Netherlands: “A civil partnership concluded on the basis of the provisions of the Civil Partnership Act 2004 is recognized by the Dutch authorities.” src
• Norway: “British civil partnership is equivalent of the Norwegian civil union/marriage.” src
• Sweden: “If you register your civil partnership in Sweden it will be is regarded as a Swedish marriage.” src
• Switzerland “Civil partnership that took place in the UK is recognised in Switzerland. If one of the two partners is Swiss, the partnership has to be registered with the Swiss authorities” src

US

• Massachusetts recognises a UK CP as a “marriage” src
• New Hampshire recognises a UK CP as a “marriage” src

Unclear cases/partial recognition:

• Luxembourg: embassy statement: “I would like to inform you that the British civil partnership is not regarded as the equivalent of the civil union in Luxembourg. Several enquiries on the matter have highlighted the fact that whereas the British civil partnership concerns same sex partners and is equivalent to a marriage, the Luxembourg civil union concerns same sex as well as different sex partnerships and is therefore not equivalent to marriage. A civil partner under British Law will therefore not have the same rights, for example for immigration, as a married spouse under Luxembourg law.” src

Cases where civil partnerships are not recognised:

• Australia: Victoria doesn’t recognise the CP but allows to enter a registered partnership despite being in a CP (may or may not be legal from a British point of view) src

Another list is found here.