I have incorporated the batch Lat/Lon to OSGB option in Excel. The source file was on the main Ordnance Survey website:
which is called:
coordinate calculations spreadsheet (xls).
everything is fine, but unfortunately, there is a quite serious mismatch between the result, we can get in Excel and the result we can get from i.e. gridreferencefinder.com varies around 60-80 meters, as you can see below:
Does anyone know where this problem might come from?