De acordo com o site do Aeroporto de Londres Heathrow: :
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.
Então, sim, a remoção de roupas é legal se os agentes de segurança considerarem que existe uma ameaça em potencial. Como @Tom comentou, é duvidoso que ele tenha que fazê-lo em público.