For the 10th anniversary of their label Marchesa, Georgina Chapman and Keren Craig came home. But what a crazy homecoming! They started out a decade ago in London in a studio on Archer Street, a Soho afterthought frequented by hookers and drug dealers. Tonight, they showed in the Banqueting House on Whitehall, an Inigo Jones-designed building of such ludicrously historical provenance that only an erstwhile seat of empire like London could possibly muster up such a location. Imagine what that must have been like for their families. Craig’s grandmother was hotfooting it downtown.