I'm transforming between OSGB36(British National Grid) and UTM Zone 31N(European 1950).
If I select OSGB36 grid from Earth-Europe-United Kingdom-OSGB36-British National Grid, I get a different result from selecting OSGB36 from Earth-Europe-United Kingdom-England-OSGB36-British National Grid. The difference is about 6 metres.
Also, If I transform from OSGB36 to ETRS89 using the British Ordnance Survey software (Grid Inquest), I get a different result from CoordTrans.
How can I reliably transform from ETRS89-OSGB36-UTM Zone 31N(European 1950).
Many thanks for your help