"The bridge is hollow," he said, tapping his foot on a utility hatch two‑thirds of the way along. "There's a control room at the north end; if you get into that, you can cross the Thames inside the bridge. Come – I'll show you."
When I was living in London, there were many old and forgotten spaces that I would have loved to explore. Walking through any tube station there are so many doors and I'm sure at least some of them open onto interesting spaces.