Upvote:9
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.