Well, I still think a cable car would be a bit of an extravaganza - more aimed at the sightseers than the commuters. Such seems to happen in other parts of the world.
But we cannot evade the main issue, which is that there is a bl**dy big river in the way if you want to traverse between SE London and NE London. Can't really help that! From my own experience give me a bridge rather than a tunnel, any day...

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Could the Dartford Bridge be altered so as to accommodate a segregated cycle track alongside the carriageway? Would give one stupendous view, but I suppose it would be noisy and fume-ridden. But better than the present system of waiting for a van to pick you up. From my experience
as a motorist, the main bottleneck at Dartford is the toll-booths, not the width of the tunnel/bridge.
Just a thought.
And if ferries are still running, seems the most pleasant - if not the quickest - way for a cyclist to get across.