Erection date: 1751
{Anti-clockwise starting from the south face:}
X miles from the Standard in Cornhill London, 1751
{benchmark}
VI miles 3 {or 'to'} quarters from Hyde Park Corner
X miles From the Standard in Cornhill, 1751
VIII miles 3 quarters from West-minster Bridge
The inscription seems to have been re-carved, incorrectly, at some point, introducing some ambiguity. The measurements are corroborated by Google Maps, except that Westminster Bridge is actually about a mile closer. If you are into milestones this is the site for you: Metadyne.
Site: Sheen war memorial + milestone (2 memorials)
TW9, Sheen Lane
The milestone is to the left of our photo, behind the traffic light post.