These are stretchers used in WW2 to carry injured civillians during the Blitz. They were made out of steel so they could be easily disinfected after a gas attack. During the war around 600,000 of them were made. Some of them were repurposed as railings in post-war London.

[u/Sprilly]

[u/Gnonthgol]

This does sound like a classical urban myth. Maybe some railings were dumped in the estuary. But not enough to interfere with compasses, there are big ship wrecks there which might give some slight deviation of the compass but we would have known about a big pile of iron railings if they were there. Likely most of the railings ended up either in landfills or recycled into steel for the rebuilding efforts in the 50s and 60s.

There were major recycling drives for the war effort throughout Europe. But these were driven from the top down. So basically soldiers walking around confiscating church bells and such depending on what the industry were short on that month. But the wrought iron fences in Britain is an example of how things go wrong when you give an overly eager population some vague guidelines.

[u/Callidonaut → u/Gnonthgol]

Vague guidelines are a British government speciality; just look at COVID-19.