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.
As someone who is doing disaster response consulting for healthcare and public health: I fucking love you guys. You make my job sooo much easier.
Seriously.
The surveillance you folks do is pretty much indisputable and far more incorruptible compared to everything else we do, in healthcare especially.
Very often you are my "discussion ending gun" when decision makers endlessly want me to prove their (flawed) point of view. A "nope, here are validated wastewater based numbers, you are wrong" is extremely satisfying sometimes.
Thanks folks!