The cops weren't fooled though. The headline is misleading.

Metropolitan Police received a call at 5.32 p.m. about a person who appeared to be unconscious inside Laz Emporium, the gallery on Soho’s Lexington Street

“officers forced entry to the address, where they uncovered that the person was in fact a mannequin,”

Seems like they did their job. Somebody calls the police, they have to go check it out.