Oddly, that section of the tunnel doesn't exhibit symmetry in the technical mathematical sense, even though at first glance most people's eyes/brains assume it does. It's not quite an optical illusion, more of a subconscious engineering assumption by biological brains that evolved in a more symmetrically repetitive pre-technological world.

I can't remember what business I had in London, but I managed to spend a few hours most days wandering along the Thames from Woolwich to Battersea, with detours, which was a most satisfying occupation with many, sometimes surprising, rewards. I stuck my nose into everything, of course, not only the Greenwich foot tunnel.