I THINK the monument you've seen MIGHT be a 'Coal Tax Post' - there's one on the north side of the line at Chadwell Heath, behind Crow Lane (apparently - I don't know the area). About 250 of these were put up from 1851 to mark the limits of the Metropolitan Police Area; they radiated approximately 20 miles from the centre of London, and covered all railwa lines & roadways. There was a tax on coal & wine brought into London, and these were the points at which tax had to be paid if importing such produce into London. I could be wrong, of course, but it makes sense; have a look for yourselves:
http://en.wikipedia.org/wiki/Coal_tax_post
Hope this helps. Happy New Year!