A nice video of some of the larger events happening in London in 2017. There are of course many, many more than just these, but this may give you some ideas.
Nice! But they left out a couple of our favorites---the Udder Belly festival (not unique to London) and the Tweed Run (a bike ride through London in vintage clothing, with champagne stops along the way.
My favorite has always been the Totally Thames Festival, which lasts all of September. Lots of different events. Something for everyone.