The longest bridge in London is one that you probably haven't heard of. The funny thing is that this bridge doesn't just win by a short head. It literally wins by a mile. The Queen Elizabeth II Bridge is crossing the Thames as part of the Dartford crossing to the east of the city.
If we limit ourselves to central London Bridges, then we get a more familiar winner. The iconic Tower Bridge is the champion at 65m.
The good thing about Tower Bridge is that it can actually climb the structure and walk between the two towers on the high-level walkways. And when you get there, you'll find a glass floor that lets you see down to the traffic and people on the bridge.