According to the Equality Challenge Unit, the protection afforded in the Equality Act 2010 to marriage and civil partnership only protects people who are married or in a civil partnership from discrimination. It does not protect people who are single, divorced, widowed or have dissolved their civil partnerships. Since the Marriage (Same Sex Couples) Act 2013, the marriage protected characteristic also includes those in same-sex marriages.
Equality Act 2010, Section 8: Marriage and civil partnership
If you would like to know more about the legal definition of the protected characteristic of 'marriage and civil partnership', please see the following text from section 8 of the Equality Act 2010:
(1) A person has the protected characteristic of marriage and civil partnership if the person is married or is a civil partner.
(2) In relation to the protected characteristic of marriage and civil partnership—
(a) a reference to a person who has a particular protected characteristic is a reference to a person who is married or is a civil partner;
(b) a reference to persons who share a protected characteristic is a reference to persons who are married or are civil partners.