[Talk-GB] Dorset/Wilts county boundary wrong...is there a definitive source?

[m902]

  Hello everyone...
I noticed that the Dorset/Wiltshire county boundary just north of 
Shaftesbury is shown in the wrong place in OSM. There is a road sign on 
the A350 about a quarter mile north of the junction with the A30, 
whereas the map shows the boundary crossing the A350 about a mile to the 
north.
Is there a definitive source for county boundaries that can be validly 
used to update OSM?
Thanks...Martin