I believe Edward still has administrative access to the TSN group on facebook. He could send a message to all the members.
Anyone with a group membership can post a message on the TSN facebook group.
A new facebook group also could be set up for the event and an intro message could encourage people to invite everyone they know to it, especially people in the UK, as well as help promote the event in other ways. I see there already is a fan page mentioned on the website:
https://mail.google.com/mail/?shva=1#inbox/127fd123aaa6d221 .
You might try to highlight the extremely reasonable cost of tickets in your advertisements.
You also might try to get UKTA members to post appealing flyers throughout London, especially at area universities and schools and hangouts around them (as well as faculty in relevant areas of study). Hipsters and artists also would be good demographics to reach - anyone with high openness to experience would make a good outreach target.
I got off of facebook a year ago because I felt I was being harassed and didn't want to make the situation any worse (shortly before a very poorly planned suicide attempt - kind of funny in retrospect). I'm wary about returning to facebook and not sure I want to deal with the distraction from GRE prep right now, though I wish I could be of more help in promoting the London H+ Conference. I wish that my situation was different in a lot of ways... :)
All the best,
Ben