That's the railway station

Putting it out there, in case someone knows the actual source of the joke.  All I have is the punchline and some vagaries of how it is funny.  

From what I can gather, it's from the early days of British Radio shows.  Circa WW1 or just after.

An English man says: "c'est la gare"  (sounds like "say la gar")

That's it.  Punchline.  

The funny thing is, that he's mispronouncing the French for "that's the war"  a common saying during the Great War.  Instead, he's saying "that's the railway station" and doesn't know it.  

It is really funny because it's the English poking fun at themselves.  At a time when most European citizens, had a smattering of English as well as their local language(s), the English were still not well educated in other languages.

So, it's been a thing in our family for the last 4 generations to say "that's the railway station" when one could say "that's the war", "that's life", or "c'est la vie" .  Like the punchline "mind you, beautifully cooked", I've never been able to find the original joke where it comes from.

Ooh, you've definitely got one there! We still use that expression; "c'est la gare" - let me try and think on it a bit!