I didn't realize that they weren't based in SF, since they have a large presence here. In fact, the Chief Legal Officer cited in that filing is based in SF.
According to statements made by the founder[1], the move to London was designed to make them more integrated into the global economy ahead of their IPO. That tells me that they aren't moving again any time soon.
[1] http://www.afr.com/technology/atlassians-farquhar-justifies-...