San Francisco. I understand attacking this city. What is important about London in the 23rd century?
Probably roughly the same things that are important about London in the 21st century. It's been described as the world's cultural capital, is the world's most visited city, and is the joint most important of the world's leading economic hubs alongside NYC. Star Trek depicts a very Western-centric view of the future, so I'd say those attributes would make London a pretty tempting target for terrorism, especially if John Harrison's goal is bringing the Earth, and not just Starfleet, to its knees.