Gender reassignment is a protected characteristic under the Equality Act 2010. It includes people who are proposing to undergo, are undergoing or have undergone a process (or part of a process) to reassign their sex by changing physiological or other attributes of sex. This includes people who are referred to as transsexual.
Under the Equality Act, gender reassignment is a personal process rather than a medicalised process. As soon as a person can show that they have reached a definitive point where they are 'proposing' to undergo gender reassignment, they are protected. Therefore it does not necessarily involve any medical treatment, and may just be that someone has made their intention known to someone or started to dress in a different way.