I was alive and reading newspapers during and after World War II, and I'm not sure I understand that Winston Churchill would have seen any irony in using a Pelikan fountain pen. But it may be that heads of state have ideas (or are believed to have ideas) not shared by ordinary people who want a good product.
From both reading and personal acquaintance with magazine journalists, I can say that photographers went right on using Rolleiflexes and Leicas and Contaxes without regard to Germany's being an enemy state. A good product is a good product.
There were Americans who for a time hesitated after the war to buy a German or Japanese car. (Well, immediately after the war Japanese products were thought to be badly made. That perception changed, to put it mildly.)
But cameras? No. And fountain pens may have belonged to a similar category. Nor do I remember people staying away from Italian films or other cultural products after the war because of disliking Fascism.
On a somewhat related theme, FPN is repeatedly told that Che Guevara wrote with a Parker 51. He was hardly a well-wisher of what he saw as American imperialism, but that didn't keep him from using a good American pen. And I suspect that Indian nationalists before and after 1947 did not see very much irony about using English fountain pens.