LONDON — There is a history-rich part of London that few people have seen, where the city braced for the Blitz, James Bond’s creator got inspiration and secret Cold War messages passed between Washington and Moscow. It’s a network of tunnels 100 feet (30 metres) below the streets that was secret for decades — but could be the city’s next big tourist destination. Local authorities have approved plans to fill the 90,000 square-foot (8,400 square-metre) site with an intelligence museum, an interactive World War II memorial and one of the world’s deepest underground bars. “It’s an amazing space, an amazing...