Dick Van Dyke definately made you cringe with his “cockney” accent.
The newest fake british accent is coming from none other than Madona. She sounds like a trailer park, bowling alley, sixth grade education bimbo trying to act refined.
That’s because that’s what she is, Archer.
I nominate Tom Cruise in that bad movie he did with Nicole Kidman. He was supposed to be Irish. My sister was in Dublin at the time for school, and said when the trailer ran, people started shouting at the screen and throwing things. 
Actually, the worst attempt at an American accent by a British actor was by one Arne Gordon in the Dr. Who episode, “The Chase.” Gordon played a tour guide at the Empire State Building and was evidently trying to go for a New York/Jewish accent. He sounded more like an alien than a New Yorker.
In no special order:
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Dick van Dyke in “Mary Poppins” sounded ridiculous.
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Kevin Costner attempted to sound English on and off for the first ten minutes of “Robin Hood,” then quit trying entirely.
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Gregory Peck was SUPPOSED to be a British officer in “The Guns of Navarone,” and his character was SUPPOSED to speak German like a native of Berlin… but he spoke both English and German with the same flat, emotionless midwestern American accent.
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I LOVE “The Quiet Man.” In fact, it’s one of my favorite movies. But Ward Bond’s Irish brogue is nothing short of pathetic.
Now, as for American actors who did English/Irish accents well, there’s Forrest Whitaker in “The Crying Game,” and the boys of Spinal Tap: Chris Guest and Mike McKean.
I thought Richard Gere’s on + off British accent in First Knight was pretty bad.
Costner definitely attempted a British Accent for “Robin Hood”. Listen to his attempts at saying “Robin of Loxley”…like fingernails on a chalkboard.
I have to nominate Kenneth Branagh for BEST American accent in “Dead Again”. It was spot on.
How about the butler from the TV show the Nanny. He does OK with for a yank
I’m showing my age again, but wasn’t Taras Bulba the movie where Tony Curtis said:
**Yondah is the castle of my faddah**.
I don’t know if an accent was even intended for the character, but if it was, it sure wasn’t Queens English.
It’s the Lucky Charms leprechaun. I erred in my earlier post. I apologize to elves everywhere.
I’d also go with with Big Dick in Mary Poppins but watch out for Dialectic Daughter of Dick …what is Madonna doing with her voice. She needs ele-ctro-cution lessons, very quickly - or that kids doomed.
Not a British accent, but…
…a few years ago, I was channel surfing and hit upon some Sylvester Stallone movie, the name of which I can’t recall. I watched it for about twenty minutes, and had to change the channel when he said the line, “I’ve never been out of California in my life.” It was hysterical - he sounded like he hadn’t been out of New Jersey in his life!
BTW, I agree with Girl Next Door about Kenneth Branagh’s perfect American accent. Dead Again was the first movie I saw with him, and I simply assumed he was an American. I was astounded when I saw Henry IV.
Well, it sounded a LOT like what someone would hear in Flushing or Astoria, and that’s “Queens English” isn’t it?
I wondered for years why exactly Disney chose Van Dyke for the role of Bert - after all, the other principals, the Banks family and Mary herself, were the genuine article. I finally determined that Walt was afraid we Americans wouldn’t understand a real Cockney. Unfortunately, the result was an offense to British and American ears alike.
I’m not a huge Gwyneth Paltrow fan, but I thought she handled the accent quite well in both Shakespeare in Love and Sliding Doors.
Peter Sellers is my choice for Best American Accent by an Englishman.
As for the Python accents, astro - you might want to look up the word “parody.”
Worst Irish accent was Ryan O’Neal’s in “Barry Lyndon”, followed by Ned Beatty’s “now you hear it, now you don’t” accent in “Hear My Song.”
I think Tim Roth does the most convincing American accent among British actors.
AuntiePam- “Yondah lies the castle of my faddah” is from “Son of Ali Baba.”
His on/off Irish accent in The Jackal was equally atrocious.
New Category: Worst American accent by an American actor
George Clooney in The Perfect Storm. I’m not an American, but there’s no way you’ll convince me that this character spent his whole life in a little Massachusetts fishing village.
I’d like to nominate Julia Roberts for her astounding work in “Michael Collins”. What was that all about?
For best American accent by a Brit, don’t forget Bob Hope.
For special consideration, I give you Cary Grant, for dropping his original English accent and adopting an inflection of his own invention. To quote Jack Lemmon, “no-one talks like that!”
Everyone: The “Magnum” guy is John Hillerman, who is actually from Texas
Kyla: It was Henry V, and personally, I thought his American accent sounded too forced in DA. His former and current squeezes both have better American accents IMHO.
Eve: We agree again. Fun movie, but “Bert” never ceases to drag MP into the realm of unbearable whenever he pops up
jayron - you got it. 
kawliga - thanks! (Your nickname - Hank Williams fan?)
kawliga writes:
> AuntiePam- “Yondah lies the castle of my faddah” is
> from “Son of Ali Baba.”
Really? I recall being told it was from The Black Shield of Falworth.
It pains me to think of this, because the man is physical perfection to me, and the programmes are my current obsession, but … David Boreanaz who plays Angel in Buffy the Vampire Slayer, and Angel. Most of the time he has a perfectly lovely accent, but for flashback scenes they take him back to his (ahem) native Galway. And dear lord, it’s painful. In fact, endearingly so.
Other than that, I’d also vote for Keanu Reeves in Dracula. (“carFayx aybeeee” was where he was headed, IIRC).
On the positives, I’ve always been impressed with Gwyneth Paltrow’s English accent. My husband thought she was English when we saw Sliding Doors.
Char “to be sure” ley.