After chewing on the idea a bit more, I see a couple of reasons that my "Build-and-run-a-Drupal-Gardensish-site for London NGOs" is not a good project for us to take on.
As someone pointed out at the meeting, Drupal Gardens already exists, as does Wordpress.com which is the default choice for low-budget non-profit websites. (I use it myself for www.postcarbonlondon.ca). So a Drupal solution would have to have compelling benefits along the lines of shared calendar/events, cross-group content sharing, or other goodies... There would need to be a compelling reason for NGOs to use our "LondonSpark" site over other free options, and building and running a site that would be compelling enough would be out of scale for our group. We need to start smaller.
So, I'm hoping you all have ideas for good one-off projects that we might want to adopt. Anything come to mind?