You would think that in a group of well-known flowers like the irises, a name like "marsh flag" would be easy to track down. But, no.
In the US, "marsh flag" is a very rare alternative common name for the (American or northern) blue flag, Iris versicolor
, Amerikanische Sumpfschwertlilie, but obviously, it's blue.
On the UK side of things, the link in #15 is by a photographer named Patrick Mackie. Elsewhere, Mackie has said, "This is a close-up of the stalk of a yellow flag iris, which I've always called marsh flag." https://patrickmackie.wordpress.com/2014/05/2...
That's about it when you Google "marsh flag". There is an 1823 book on Google Books, Flora Domestica: Or, The Portable Flower-garden
by Elizabeth Kent that uses "marsh flag" as the translation for a French common name for the English "Common Yellow Iris", which I think is probably Iris pseudacorus
A gladiolus like Sumpf-Siegwurz is very remotely possible -- "flag" isn't exclusively
reserved for irises, but it almost is. And most of the other plants called "flag" belong to the genus Acorus
, the sweet flags, which all grow in wetlands, particularly "marshes", but their flowers are greenish & inconspicuous. So I would exclude both as more unlikely than likely.
Sumpf-Schafgarbe / Achillea ptarmica -- grows in England, but no. Sorry.
If "weiß" can possibly refer to something else (#12), then Marianne (BE) most likely got it right first.