When it comes to English ".....lowest tier NON-'entry level O/U & SxS?" one name comes to mind William Baker of Birmingham. He was responsible for design of Vickers Imperial side-lock and Over/Under shotgun using patents 4922, 9768 and 34926. The level of finesse depended on what company finished up the final product. For example O/U from Harrison & Hussey would be very fine gun.