Just declaring a transgender identity doesn't actually fulfil the requirement of the EA2010 though:
'Gender reassignment
(1)A person has the protected characteristic of gender reassignment if the person is proposing to undergo, is undergoing or has undergone a process (or part of a process) for the purpose of reassigning the person's sex by changing physiological or other attributes of sex.'
However, without defined terms for 'proposing', 'physiological' and 'other attributes of sex' we're in the dark as to what qualifies as proposing (if you're already on hormone treatment or have had surgery / are booked for surgery, then it's obvious you fulfil the criteria).
Have people taken the safe view to date, that simply saying you're transgender is enough, to make sure they don't fall foul of poorly-defined legislation, or does case law support that?