All this talk about Wales reminds me of something. I may have heard this exchange in a movie or sitcom.
Character A: “Character B talks/behaves oddly because reasons.”
Character B: “Oh. I just thought he was Welsh.”
Or it might be another alternate-ending-to-Big that only exists in my head. Anyone?
But not this, from FAMILY GUY?
No, not that. (And of course, I meant character C talks/behaves oddly.) I’m thinking of an Englishman (or perhaps a Scot) accepting Welshness as a blanket explanation for all kinds of strange habits.
Here’s a video of 15 minutes of Jimmy Carr insulting Rhod Gilbert, who is Welsh. Pretty funny, at least the first part.
Blackadder Goes Forth - episode ‘Major Star’
Captain Blackadder : There’s something wrong with your fiancee.
General Sir Anthony Cecil Hogmanay Melchett : Oh my God, she’s not Welsh, is she?
Bloke1 in pub: Everyone Welsh is either a prostitute or a rugby player. 
Bloke2: My wife is Welsh!
Bloke1: What position does she play?!
In one of Dick Francis’ novels, a character is jokingly referred to as “leeking like the Welsh”.
I know the leek is considered a national symbol of Wales, but never quite understood the remark.
I think it’s a punchline in the same sense as any “joke” along national, ethnic, or similarly identitarian lines.
ETA: Which is to say it’s probably common enough that it won’t by itself narrow it down to a film. If memory serves, an early (or at least middle) season episode of Family Guy even made a “Welshman” joke.
ETA2: Season 3, so early. Link spoilered because, well, I mean it’s a joke about national identity…
I had no idea of the kind of contempt that Welsh people had endured from the English until reading this fascinating book:
Neighbours from Hell?: English Attitudes to the Welsh by Mike Parker
The author is an Englishman who relocated to Wales to write a travel guide.
Could the exchange @Rilchiam is thinking of have been in the movie “Notting Hill”, where Rhys Ifans plays Spike, Hugh Grant’s eccentric, and Welsh, flatmate?
English friend told me about taking his son to a soccer game in Wales. They started by playing the national anthem of the visiting team, in this case England, which was at the time God Save the Queen. The visitors side of the stadium stood up during the anthem while jeers and boos emanated from the home side. The situation reversed with.jeers from the visitor’s when the home team’s anthem was played, also God Save the Queen.
A very drunken bloke noticed two rather large women at the bar conversing in a different dialect.
“'scuse me, are you ladies from Ireland?”
“Wales, you sot!”
“Beg pardon, are you whales from Ireland?”
He woke up in the alley.
Mmmmmmaybe. Although that specific punchline doesn’t work if the guy is Welsh.
This doesn’t sound right. Our (Wales’) national anthem is the beautiful and moving Mae Hed Gwlad Fyn Hadai (The Land of My Fathers) and is sung prior to all sporting events Wales are playing in. Undoubtedly so if playing England.
As an aside, I read some years ago that it is in fact the first anthem ever to be sung at a sporting event in IIRC 1905 when Wales played New Zealand at rugby. The New Zealand team performed the Haka just before ko as is their tradition, and the Welsh team thought they should do something in return and sang the anthem.
MiM
Sorry, all I had was the anecdote. Was he off about the sentiment as well?