Where is it? Woolwich, out east along the Thames path.
What’s quirky about it? That rare thing, a free ride: as the link between the North and South Circular, the ferry shuttling over the Thames between North and South Woolwich has to cost you nothing. So there is, after all, such a thing as a free launch. Your nearest other free ferry of this size is at Torpoint in Cornwall.
Why bike there? Bikes can head straight to the front of the road queue, and be first on the car deck when the boat docks. If you you join the ferry with the foot passengers, you’ll have to schlepp your bike up and down some stairs. At times when the ferry isn’t operating, or just for something different, push your bike through the foot tunnel just downstream, similar to Greenwich’s (but slightly longer, at 500m). Look for a pointy, round brick hut.
The ride along the Thames Path between here and Greenwich goes past the Thames Barrier, unsettlingly like alien landing craft, and the O2 dome. There’s a curious optical illusion as work as you do so: the three skyscrapers in Docklands (HSBC, Citigroup, and ‘Canary Wharf’ or 1 Canada Square) appear to swop places as your river path winds round.