We are going to have issues where there will be men in women's changing rooms, in leisure centres, for example, and staff won't be able to do anything because they will say that their passports prove they are female.
I'm not sure that the Draft Code explains the legal position sufficiently robustly, what with all the pussy-footing around regarding whether sex is special category data.
It is not illegal sex-discrimination, and it never has been, for a provider to exclude, from a single-sex service, someone who looks like they are not the sex for which the service is provided, irrespective of what documentary or other proof they offer as proof of sex. Because Schedule 3 provides immunity from liability for perceptive sex-discrimination.
It is illegal gender reassignment discrimination to exclude a transman from a women-only service, or a transwoman from a men-only service, other than as a proportionate means of achieving a legitimate aim. I don't believe this will be hard to police, even without documentation, because a) transmen are not dicks, and b) unicorns transwomen who pass will be trying it on in the Ladies, not the Gents.