From my understanding your colleague committed a crime under the Indecent Displays (Control) Act 1981 and you can refer the matter to the police - which I would strongly recommend as this is beyond an employee-employer relationship.
And it brings the employer into a position that the company is forced to make sure that the offender cannot reoffend against anyone (not just you). While the first offense is nothing the company can really be held liable for, anything after they have (officially) made aware they can be held liable for.