From the beginning of Dickens' Barnaby Rudge

"In the year 1775, there stood upon the borders of Epping Forest, at a distance of about twelve miles from London—measuring from the Standard in Cornhill,’ or rather from the spot on or near to which the Standard used to be in days of yore—a house of public entertainment called the Maypole; which fact was demonstrated to all such travellers as could neither read nor write (and at that time a vast number both of travellers and stay-at-homes were in this condition) by the emblem reared on the roadside over against the house, which, if not of those goodly proportions that maypoles were wont to present in olden times, was a fair young ash, thirty feet in height, and straight as any arrow that ever English yeoman drew."

From the Internet:
"London has never had a Milliarium Aureum but the next best thing was probably an old water tank whose former position was the datum for many of the milestones on the turnpike roads.
The Standard in Cornhill, was a water cistern with four spouts, made by Peter Morris, a German, in 1582, and supplied by lead pipes with Thames water. It stood at the east end of Cornhill, at its junction with Gracechurch Street, Bishopsgate Street, and Leadenhall Street. The water ceased to run about 1600, but the Standard itself remained until 1674, the year before the publication of Ogilby's road maps were published with their mileages measured from the Standard."