o know the reason that Whitminster records are included in the Forest of Dean FHT indexes. I know that there will be a good reason but not being a local,

Its simply one of geography - there was a reasonable flow of people across the Severn esp. Newnham to Arlingham peninsula - the Great Horseshoe Bend - and Wheatenhurst is the Registration district :-)

It is believed that the Romans built a road to Arlingham and beyond