Coordinate conversion methods between England's Highways Standards and British National Grid (OSBG36 or EPSG:27700) in pure Python.
The module can be installed through pip by running pip install hsgconv. The example shows the recommended way to proceed and shows cumulative errors.
Read the Docs: link
Note: Tested on Python 2.7 and IronPython 2.7 - should work on 3.x too.