OSGB mesh is used very often in China. Many visualization platforms, touch-up tools are based on OSGB. BIM user get the model from the market, or after touch-up, they want to import it into MicroStation or ORD for the following workflow. I know FME has a tool to convert OSGB to FBX, but didn't try by myself.
As 3mx is public format it is possible using this opensource tool - https://github.com/ProjSEED/lodToolkit
Afterward 3mx can be converted to 3sm in CCEditor or Descartes