THIS STONE IS A REMNANE OF THE LONDON BRIDGE DESIGNED BY SIR JOHN RUNNIE
THE BRIDGE SPANNED THE RIVER FROM 1831 TO 1967 IT WAS THEN SOLD TO ROBERT P. MCCULLOCH AN AMERICAN ENTREPRENEUR AND TODAY CAN BE FOUND IN ARIZONA, USA

The stone is a remnant of the New London Bridge (the bridge that replaced it and is the current bridge is called "Modern London Bridge").
The New Bridge existed between 1831 and 1967 and, as indicated on the sign, was sold to the American entrepreneur Robert McCulloch for a sum of approximately $2.5 million. Before the bridge was dismantled, each of its stones was marked, so that they could be reconstructed later in Arizona, United States. In its place, the bridge that exists today was built Click for sign's details

The steps and arch that led to the previous bridge are commemorated on a separate sign Click for sign's details