Assuming that the milestone is actually included in the national database, you should be able to locate the milestone by using the place name based search facility in Google Earth.

However, you need to decide firstly which regional file to look in. This is not always obvious as we have based the regions of England on the pre-1974 counties classification. In some cases there were major realignments of county boundaries in the 1974 revision, one of the most well known being the district of Saddleworth near Oldham moving from Yorkshire to Lancashire.

If the milestone you are looking for is close to a regional boundary then it would be better to load all the neighbouring regional files. Once you have found the milestone, the 2 letter prefix of its National ID will tell you which old county and hence region it belongs to.