Is it legal for airport officers to ask someone to remove their jeans if they're setting of metal detectors?

I'm travelling soon and one of my young friends told me that he had to take off his jeans and only remain in underwear last week in London Heathrow when departing for a flight because there was some metallic piece that kept on setting of the detector. They asked him to take of his jacket, empty his pockets until they decided he had to take them off.
Now, I'm not sure whether he's exaggerating or not but I am concerned about this. It doesn't seem that they offered him to be screened in a private room and just in public.
Is this legal or illegal?
Best Answer
According to London Heathrow Airport's website:
The Department for Transport considers that there are no known health effects from the scanners in use at Heathrow. The only alternative that can be offered to a scanner is a private search which allows for a more extensive hand-search than usual. Passengers will be escorted to a different location in the airport from the main search area (eg a private search room). The private search may involve the loosening and/or removal of clothing. A person undergoing a private search may ask to be accompanied by a witness.
So yes, the removal of clothes is legal if the security officers deems there to be a potential threat. As @Tom commented, it is doubtful he had to do it in public.
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