Well in this case "To be legally recognised as a transgender woman, a person must have been diagnosed with gender dysphoria and lived as a woman for two years."
So better get started on that in advance.
what does it mean 'live as woman for two years'? What if I am gender fluid?
Well found this
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/720834/t450-eng.pdf
That part isn't too onerous, less so next to the diagnosis.