In common with the LBSCR, the SER erected mile posts based on a zero point (datum) of London Bridge. This is clearly demonstrable by looking at OS 1:10,560 maps from the 1870s and 1880s. The SER's mile post mileages went via the original route (Redhill) and the OS maps show this to be still the case at Merstham, Dover, Rye and Winnersh in the 1870s.
When the line through Polhill and Sevenoaks tunnels was opened in 1868 it, too, was mile posted from London Bridge and there was a 'change of mileage' at Tonbridge with the 'old route' prevailing for mileages onward to Ashford etc. Again, this is evident from the OS maps.
Remiling long stretches of railway is an expense that many railways avoided but at some point in time the SER changed their datum to Charing Cross, line opened 1864, but the map evidence indicates that neither this opening nor that of the more direct route through Chelsfield triggered the change, at least not immediately.
1. When was the change of datum from London Bridge to Charing Cross made.
2. Why was it changed.
3. What mile post mileages applied between Charing Cross, Cannon Street and London Bridge prior to the general adoption of Charing Cross as the datum point.