World Cube Association: The official governing body for speedcubing competitions. Contains results from all competitions, and the world rankings and records amongst other things.
Speedsolving Forum: The largest online speedcubing forum and is predominantly English speaking. Cubers across the world use this site to discuss everything related to cubing and it is used by many UK cubers. This thread is a good place to discuss anything related to the UKCA, although competitions usually have their own threads.
Speedsolving Wiki: Contains a large amount of information on a variety of topics, including many lists of algorithms.
London Cube Meets Facebook Group: Post once you join, because the meetups often get arranged outside this group and you may never find out about them if you just sit back and watch.