Are transgender employees protected against discrimination if they have not undergone medical treatment to reassign their gender?

Yes, the definition of gender reassignment under the Equality Act 2010 does not require a person to be under medical supervision in order to come within it. The definition covers people who are "proposing to undergo" a process for the purposes of reassigning their sex. The Equality Act 2010: code of practice on employment advises that a person does not have to have reached an irrevocable decision to reassign their gender. It states that people who have started a gender reassignment process but have withdrawn still have the protected characteristic because they have undergone part of a process of gender reassignment.