It's definitely very optional here (at SE). Some people just grab a plate of food and go back to their office/desk/secluded corner to enjoy it by themselves or with a smaller group.
The Stack Exchange office has a number of open areas where folks can (and do) work if they bring their laptop along. As a developer, though, I like my multiple monitors, my keyboard, etc. Once in a while a change in the environment is nice, but most times I find not working with my optimized setup is more annoying than anything.
(Mind you, since our teams are distributed, there's often no such thing as "working together in a small-group environment" that doesn't involve being on a hangout, and everyone being in a private office helps immensely with that.)
We do have UK presence now. :) I don't know how these things worked out with other people before that, but I can speak about myself.
I'm one of the community managers at SE and I work from my home in Canada where Stack Exchange does not (yet?) have an office. My arrangement is basically that I send in an invoice every month like any contractor would and it is up to me to take care of my own taxes and so on.
