These first two bridges, the Pontcysyllte aqueduct (opened 1805) in north Wales and the iron bridge (opened 1781) at Ironbridge, were part of an industrial archaeology series of stamps published in 1989. At the time, the bridge was already a UNESCO World Heritage Site, then, in 2009, the aqueduct was added to the list.
I don't know if this third bridge, the Gateshead Millennium Bridge will ever join them on the list. Only time will tell. The stamp was part of the huge series issued to mark the Millennium, this one in June 2000.
This is a post for Sunday Stamps, hosted by Violet Sky at "See it on a Postcard!" As usual, click on the link to see some bridges on stamps.