> This is additionally confounded by the fact that British people really do mean "sarcasm" when they say "irony."

I've never experienced this in actual conversation with British and/or other non-US English speakers. But the internet definitions I'm finding seem to all include 'sarcastic but somewhat subtle' in the definition of ironic. For me the definitions of sarcasm and irony do not intersect. Sarcasm is intentional whereas irony is at best random or more often a signifier of implied 'fate'.