Can I use the bathroom I feel most comfortable using at work?
Whilst it is not a legal requirement, employers are encouraged to support your choice in using the toilet facilities appropriate to your preferred gender in the workplace.
Gender Diversity Policy Guide for Employers:
“a trans person should be free to select the facilities appropriate to the gender in which they present. For example, when a trans person starts to live in their acquired gender role on a full-time basis they should be afforded the right to use the facilities appropriate to the acquired gender role. Employers should avoid discriminating against anyone with the protected characteristic of ‘gender reassignment’. Where employers already offer gender-neutral toilets and changing facilities, the risk of creating a barrier for transgender people is alleviated.”