From the newly-updated Times style guide:
Sex change. Avoid this inaccurate and potentially offensive shorthand for the process of gender reassignment by surgical and other medical procedures. Note that from a legal perspective, people in the process of gender reassignment should be called by whatever name and honorific they prefer, and that the appropriate personal pronouns reflecting that change should also be used. The Gender Recognition Act 2004 makes it an offence to identify a person who has been granted a gender recognition certificate by their previous name or gender. Note that transgender and transgendered (adjectives, never nouns) are terms used by people with this condition and by the medical profession in preference to the older term transsexual.