Same sex couples have been able to form a civil partnership since 2005. This is a legal union which entitles the people who enter into it to similar legal treatment in terms of inheritance, tax, pensions and next-of-kin arrangements as in a marriage. To date opposite sex couples cannot form a civil partnership and have minimal rights as cohabitees unless they marry. In June 2018 the Supreme Court unanimously agreed that different sex couples should be able to enter into a civil partnership, and failure to allow this to happen would be an abuse of their human rights. The Government has been urged to give consideration to an amendment of Statue to allow this to happen.
However until the law is changed cohabiting couples can draw up a cohabitation agreement which deals with the financial aspects of their relationship, but this is a different situation to a civil partnership and does not carry the same legal rights.
If you are considering entering into a cohabitation agreement it is appropriate to obtain legal advice concerning the documentation that will be required. If you are wishing to enter into a civil partnership then legal advice should be sought.