We have 12 people currently signed up without any particular publicity - I was planning on mentioning it some more in the new year. I'm not too worried about having people attend. Even if there are too many gamejams and hackdays about these days.
(It's quite possible the space will have doubled in size by this point anyway, which would allow a lot more space. But I'm waiting for this to happen before I rely upon it happening.)
And I have now filled out the form to register it as a center - it needs approval before it will go live, but will then be at http://globalgamejam.org/sites/2011/london-hackspace
. I'm not all that fussed about it, but I don't see a downside to attracting GGJers.