How can I negotiate a free conference venue in London?
My answer to How can I negotiate a free conference venue in London? on Quora
The two top options in my experience are universities (including student unions) and private companies with good meeting facilities.
In both cases, you will need a strong internal champion within those organizations. For universities a good option here may be a student union society of some sort. For companies, an employee who knows how to book rooms, work with the facilities team etc.
It goes without saying that your event will need to have a theme that strongly resonated with the group that you are asking to host it for free.