The law, called an act, making Sexual
Orientation Discrimination illegal in the
workplace came into effect in December
2003, it is the:

Employment Equality (Sexual Orientation)
Regulations 2003

(Statutory Instrument 2003 No. 1661)

The exact act can be found online here

The following groups are covered in the act:

Lesbian / Gay - Attracted to the same sex
Bisexual - Attracted to both sexes
Heterosexual - Attracted to opposite sex

OR those associated with anyone who identifies himself or herself as one of the above. For example having a gay friend or family member

OR those who are wrongly perceived; for example a heterosexual "effeminate" male who is harassed, as he is wrongly perceived as homosexual.