I believe you should be able to form a civil partnership in the UK as foreigners, regardless of it being same-sex or not.
You can only get a civil partnership as a same sex couple.
There's some subtle differences being made between marriage and civil partnership, but none of these should matter to you if you don't intend to stay in the UK.
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 bullte point there should be your case, but do visit the two linked pages and find all the detailed information there.