There have been two batches of tickets so far, and it’s unclear whether more will follow so keep an eye on the official website. You can also subscribe to the following pushbullet channel to get notified whenever a new article is posted to the techcrunch RSS feed that contains the word “hackathon”: (Thats how I got my tickets.) https://www.pushbullet.com/channel?tag=hackathontcdisrupt
Official description: Preceding the Disrupt Conference is Hackathon weekend on December 5-6, where developers and engineers descend from all over the world to take part in a 24-hour hacking endurance test. Teams join forces to build a new product, present it on the Disrupt stage to a panel of expert judges and an audience of tens of thousands and compete for a variety of prizes, including the chance to win free tickets to the Disrupt Conference. Products created at the Disrupt Hackathon have seen great success beyond the event, like GroupMe which was created overnight and ultimately acquired by Skype for $80M.
Official website: http://techcrunch.com/event-info/disrupt-london-2015/hackathon/