Basically, if the Brits use "best", it refers to sidelocks--other than a few cranky folks like Mr. Greener who specialized in boxlocks. But I think one can refer to a "boxlock best" as being one with all the bells and whistles, although not a sidelock. One could include top of the line Parkers, Foxes, Ithacas, and 21's in that "boxlock best" category, even though we don't generally do that.