In fact, "deadnaming" is mandated in the Gender Recognition Act 2004 in specific circumstances:
Section 12: Parenthood
The fact that a person’s gender has become the acquired gender under this Act does not affect the status of the person as the father or mother of a child.
[ie: name and sex of parent cannot be altered on any child's birth certificate]
Section 20: Gender-specific offences
(1) Where (apart from this subsection) a relevant gender-specific offence could be committed or attempted only if the gender of a person to whom a full gender recognition certificate has been issued were not the acquired gender, the fact that the person’s gender has become the acquired gender does not prevent the offence being committed or attempted.
(2) An offence is a “relevant gender-specific offence” if—
(a) either or both of the conditions in subsection (3) are satisfied, and
(b) the commission of the offence involves the accused engaging in sexual activity.
(3)The conditions are—
(a) that the offence may be committed only by a person of a particular gender, and
(b) that the offence may be committed only on, or in relation to, a person of a particular gender,and the references to a particular gender include a gender identified by reference to the gender of the other person involved.
[for "gender" read sex].