This projection is specifically used to convert between ETRS89 and Ordnance Survey British National Grid using the OSTN97 transformation.
The National Grid Transformation OSTN97 is an interpolated grid of horizontal plane shift parameters covering GB at 1-km resolution, which converts ETRS89 GPS coordinates to National Grid coordinates with an accuracy of 20 cm (RMS). It takes no projection-specific parameters.
Note: Users should not normally have to use this projection to define their own systems – FME ships with the OSGB-GPS coordinate system, which is based on this projection.