London is surrounded by Coal Duty Markers used for taxation in the past, I'll go an find them if I am close by.
There were originally around 278 posts, obelisks and plaques encircling London, 211 still remain though many have been resited.
A complete list and more information is available on Martin Nail's marvelous site.
Marker 45
Obelisk at TQ1165796562 Relocated to TQ1164396578
Marker 46
Post at TQ1160996252 Relocated to TQ1161996250
Marker 47
Obelisk at TQ1187395500 Relocated to TQ1102896393
Marker 48
Post at TQ1196295266 Relocated to TQ1101996382
Marker 52
Obelisk at TQ0917692069
Marker 54
Post at TQ0836092324
Marker 55
Post at TQ0784192336