We're mostly a place to find out about hackathons happening across the UK that you might want to join in over the next few months. Usually there is a 3rd-party organiser's website for signup and more information. Please always sign up on the organiser's website because your RSVP on Meetup.com/UK-Hackathon-and-Jams will not be sufficient to get you in.
Watch this 5-minute talk to see what a hackathon is and what you do there.
Thinking about organising a hackathon? Read the Hackday Manifesto for the very best advice.