From Girlguiding site: ( i was interested to see what they actually say as i'm a current volunteer)
"One of these nine protected characteristics is sex (whether you are male or female) and another is gender reassignment, which in loose terms equates to being trans.
Gender reassignment is a self-identified characteristic– there is no need for anyone else to ‘confirm’ someone is trans, or for the trans person to have started any kind of physical or other transition attributed to sex, such as clothing. There is also no need for a Gender Recognition Certificate (GRC) for those over age 18."