As I remember there were actual poppies at the tomb of the unknown soldier in Westminster Abbey, at least in the early summer, several decades ago. At the time I was fond of G&S's Patience, and wanted to walk down Piccadilly with a poppy (or a lily).* I could find no florist that sold poppies (or, alternatively, lilies). Obviously the tomb wasn't a viable source; but I'm pretty sure it did have them.

* Then a sentimental passion of a vegetable fashion
must excite your languid spleen,
An attachment a la Plato for a bashful young potato,
or a not- too-French French bean!
Though the Philistines may jostle, you will rank as an apostle
in the high aesthetic band,
If you walk down Piccadilly with a poppy or a lily
in your medieval hand.