Its indeed the Warley Museum in West Yorkshire.

 

A bit more about it:

Several museums lay claim to the title of “world’s smallest”—the MMuseumm in New York City is housed inside an elevator shaft, while a teeny converted shed in the town of Superior, Arizona calls itself the World’s Smallest Museum by coming in at just 134 square feet.

 

However, the northern English town of Warley, West Yorkshire, about 25 miles southwest of Leeds, has turned one of those iconic red phone booths into a mini-museum. Inside its four thin, bright-red walls, The Warley Museum shows off memorabilia from the small town’s history, including old photographs, antique jewellery, and glass etchings, and will rotate its exhibits every three months.

 

While the Warley Museum is being billed by residents as the world’s smallest, locals would like to make that designation official. The Telegraph reports that the Warley Community Association (WCA) has formally applied to the Guinness Book of World Records, hoping that their little red museum—which, like any other non-Tardis phone box, can only fit one visitor at a time—will make it into the big leagues.