Your employer is not allowed to discriminate against you by less-favourable treatment, if you complain about such discrimination, or if you give evidence that a person has been discriminated against.
Who is protected
You are protected by this regardless of your sexual orientation, whether lesbian, gay, bisexual, or heterosexual.
You are protected by this if you are being discriminated against or harassed because your employer or your co-workers think they have identified your sexual orientation, even if they have got it wrong.
You are protected by this if you are being discriminated against or harassed because of the sexual orientation of your friends or family.
Who is not protected
If you identify as trans or polygendered or intersex, but have never entered gender reassignment, you may not be protected by the regulations against discrimination on grounds of gender identity. (Gender reassignment is defined as a process undertaken under medical supervision for the purpose of reassigning a person's sex by changing physiological or other characteristics of sex. This is a wide definition, which should include people who have non-surgical treatment.)
Further information for LGBT people for on employment and discrimination:
Transgender Good Practice: Employment Issues
Equality Network > Your Rights > The Law > Discrimination