This marker is on National Cycle Network Route 1 between Dover and the Shetlands. It is just a few yards from Tripcock Ness at Thamesmead. It shows 7 miles to Greenwich and 1191 miles to Inverness to the left and 10 miles to Dartford and 106 miles to Dover to the right. For more about the National Cycle Network and a map see https://en.wikipedia.org/wiki/National_Cycle_Network
This is also on the Thames Path.