{On the stone laid into the paving:}
This stone is a remnant of the London Bridge designed by Sir John Rennie. 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 long piece of text on the river wall below the information panel reads:}
There are two things scarce matched in the universe: the sun in heaven and the Thames on earth. Sir Walter Raleigh, 1552 – 1618.
The long piece of text is attributed to Raleigh, here and all over the web, but we can find no source for it, so we've put this page in our Puzzle Corner.
2022: Edward Kegg found a reference to the source in a letter from E. L. H. to the 1863 'Notes and Queries'. Walter Raleigh is a character in Walter Scott's 1821 historical romance 'Kenilworth'. In chapter XV Scott puts this quote into Raleigh's mouth. So really it's a quote from Scott, not Raleigh at all. Puzzle solved. Thank you, Edward.
Site: Jubilee Walkway + London Bridge (2 memorials)
SE1, Cathedral Square
In our photo the pavement plaque is between the two groups of seating, about where the person reading the information panel, is standing.
No sooner did we collect and do all the research for the information panel than it got removed. In June 2022 it was replaced with a panel about the June 2017 terrorists attacks on London Bridge. We will publish that one soon.
There are plans to re-erect the information panel, in this square, but it's not clear when.