Speakers' Corner
Speakers' Corner is located at the north-east corner of Hyde Park, near Marble Arch. Note that it is a different corner from Hyde Park Corner. It is a long-established public forum, dating back at least a century and a half before it was (as is more commonly known) designated in an act of Parliament in 1872.
Although one or two soapbox campaigners can be found there at almost any time it is most active on a Sunday, when a wide selection of cranks and visionaries are joined by large crowds of listeners and hecklers. It is true that many of the attendees are tourists and regular speakers are often uninteresting drones, but Sturgeon's Law applies as ever. Interesting things do happen here.
Resonance FM has a Speakers' Corner radio show, and the site features a history of Speakers' Corner.