The media is constrained by the Editors Code of Practice. Not adhering to this attracts hefty fines and the obligation to carry a prominent apology if a complaint is made and the newspaper is found to have breached the code. Here is the relevent section:-
- Discrimination
i) The press must avoid prejudicial or pejorative reference to an individual's, race, colour, religion, sex, gender identity, sexual orientation or to any physical or mental illness or disability.
ii) Details of an individual's race, colour, religion, gender identity, sexual orientation, physical or mental illness or disability must be avoided unless genuinely relevant to the story.
So The Mirror decided that Lauren being transgender was 'genuinely relevant' but the BBC and The Guardian have erred on the side of caution and decided to avoid mention. Though they have decided that mentioning Lauren's mental health or disability is OK.