The Hillworth Road bridge over the former Berks Hants extension railway in Devizes still shows a distance of 85 miles on it Where was this distance measured from?

The Berks & Hants Extension Railway was part of the GWR when the line to Devizes from Hungerford was opened in 1862. Hence the distance of 85 miles shown on the Hillworth Road bridge would most likely be the distance from Paddington Station in London. The pre-motorway road distance to London was, I believe, 87 miles (presumably measured from Charing Cross) and would support a rail distance of 85 miles from Paddington.