Ed,
I had no idea that Marble Arch partly comemmorated Nelson, but then read that is was also supposed to honour Wellington, although this latter idea apparently didn't come to fruition - perhaps because Wellington was still alive at the time! Even the Nelson connection would seem somewhat obscure, since there don't appear to be any ships on the reliefs and the figures are in Roman dress!
The idea that the Arch was moved from the front of Buckingham Palace, because the Queen's coach couldn't pass through it, may actually be erronous. It seems as though the more likely reason is that the Palace was resigned in 1851, to give it the flat front we know today, and the plans didn't include the Arch which of course had to be moved. Incidentally, I also read that the Gold State Coach (the big one!) actually passed through the Arch, in its present location and with the present Queen 'on board', during the Coronation procession in 1953. I wonder if they were testing the story – even so it must have been a bit of a tight fit!
