David Siegel leads blockchain workshops and gives keynote speeches. He is represented by the London Speaker Bureau. Please see his speaker page, which has a video of a talk from 2016:

http://londonspeakerbureau.com/speakers/david-siegel/

His next talk is at Google Campus in London on March 23rd. Seats are still available but will sell out. Get your free ticket now:

https://event.bookitbee.com/10322/start-booking