I think it's a story from the 60s or 70s, set in its present. A British SS from an alternative reality in which Nazi Germany has won the war arrives inadvertently to our reality's London. I remember some phrases that said something like this:
He suddenly felt uncomfortable. Sometimes his SS English Division uniform aroused angry glances of some old churchillians, but it had never happened before that everybody looked at him that way.
In the end, the various realities (not only the Nazi one and our own) come to war.