I think that some of the rendering set sets, like possibly mapnik that may use the Proj module can module. Can use proj to process a lot of projections and althogh older versions didn't have OSGB NG the latest version seem to list it. Proj is now I thing version 4.x.x.x and comes with some package servers (to link with download packaged programs that use it and from a range of download sites for the best option search for proj on your system or take a look at the requirments lists of renderers and where they sugest you should get it. I think upstream it comes out as a support file of projection parameters from a reaseach and science body mainly for use with thier s/w (of the top of my head ESRI comes to mind but could very wrong. -just looked it up it was USGS [aledgedly])
These links might help a bit..
[http://proj.maptools.org/faq.html][1]
[https://github.com/kartena/Proj4Leaflet][2]
[https://trac.osgeo.org/proj/][3]
Essentialy its a big parameter file that a program uses to self configure, and can then chunter through the conversion maths with for your chosen projection.
[1]: http://proj.maptools.org/faq.html
[2]: https://github.com/kartena/Proj4Leaflet
[3]: https://trac.osgeo.org/proj/
