It's not actually Head Office but a "committee room" in the constituency (Paddington North) for the duration of the campaign, but I can see what point you're making. But that slogan is a statement of fact not an exhortation to do something. I agree with you, though, that I can't see anything unprincipled about saying "Vote Socialist" or "Vote Socialist Party". "Vote Groves" (or whoever), however, would have been going much too far.For those unfamiliar with this old-fashioned term, here's the definition of a "committee room":https://en.wikipedia.org/wiki/Committee_room