Hungerford Bridge and Golden Jubilee Bridges
The Hungerford Bridge crosses the River Thames in London, and lies between Waterloo Bridge and Westminster Bridge. Owned by Network Rail Infrastructure Ltd it is a steel truss railway bridge flanked by two more recent, cable-stayed, pedestrian bridges that share the railway bridge's foundation piers, and which are named the Golden Jubilee Bridges.

The Hungerford Bridge crosses the River Thames in London, and lies between Waterloo Bridge and Westminster Bridge. Owned by Network Rail Infrastructure Ltd it is a steel truss railway bridge flanked by two more recent, cable-stayed, pedestrian bridges that share the railway bridge's foundation piers, and which are named the Golden Jubilee Bridges.
Check the official site before travelling; hours and prices can change.
More bridges

Tower Bridge
The bridge people mean when they say London Bridge. Walk the high-level walkway for the glass floor.

Millennium Bridge
Foot-only suspension bridge from St Paul's to Tate Modern — the only crossing aligned on St Paul's dome.

Hammersmith Bridge
Currently pedestrian-only, Victorian wrought-iron beauty — best photographed at golden hour.
Source: wiki-list