"Flag" is the common name used for a type of iris that grows with its roots in either waterlogged soil or at the edge of a pond. The leaves are sword shaped, the petals flutter in the breeze - and the day I took this picture was very windy.
I don't know what the Latin name for this plant is, I don't know what variety it is and I don't know if it's possible to buy it from a garden centre. I have only ever seen it growing in one location in the New Forest.
If anybody knows the species I would be grateful if they could share their knowledge.