Rattle off a list of them funny cuss words y’all got over there. Wank and fanny and all that.
The English chap I work with always says “Poxy tart” for me. Right after I tell him to learn to speak english.
We are pretty good friends.
You’re British?? Ooh, I LOVE British accents! Type something British here so I can see your accent!
In all seriousness, cuss them out. 
One of Rambling Syd Rumpo’s songs?
Or this BBC announcement from Python?
Or the biblical news from “BBC B.C.”?
Or go classical and use pseudo-Shakespeare from “Beyond the Fringe”.
Next time either of you is in Denver, call me. I can do a wicked Hotspur. 
I don’t do Yorkshire, but my Welsh is pretty convincing.
People here in Atlanta have said the same to me. I’m from New York.
I usually respond, “It’s YOU with the accent.”
“there’s trouble down the mill. One of the crossbeams got out askew on treadle.”
Feh. In Charlotte, it works the other way around.
“Youse gize tawk wit dat accent soze I can bailey unnastanz ya.”
How about “Piss off”?
All your base belong to us.
I was thinking “Bugger off.” Great minds…?
This is one of my favorite jokes ever. :-]
The talk show host Conan O’Brian always has British guests do an exaggerated imitation of Mary Poppins dialog “What say, gov’nor? Wan’ yore chiminey sweept?”
I’d be tempted to go with “Yippee kai yay, motherfucker” as the least British thing possible to say.
Daniel
How about memorising a bit of the shipping forecast? Sounds wonderfully evocative, and is also totally incomprehensible (audio file at the top right of the linked page):
http://search.bbc.co.uk/cgi-bin/search/results.pl?q=shipping+forecast&uri=%2Fradio4%2F
The guy’s from Yorkshire, with a northern accent. I don’t think he could do a southern accent if he tried.
How about some northern sayings?
“Wash be ‘ind thi lug olls. Tha’s got enuff muck in there t’ grow ‘taters’”
Ah yes, good point. Maybe he can sing them a bit of “On Ilkley Moor Bah T’at”, including the line “without thy trousers on” after each verse for good measure!
What you need is some good banter.
“Bally Jerry pranged his kite right in the how’s your father. Hairy blighter, dicky-birdied, feathered back on his Sammy, took a waspy, flipped over on his Betty Harper’s and caught his can in the Bertie.”
Or put on your best Scottish accent and recite:
“‘Twas in the month of December
In the year of eighteen eighty-two
The railways lines near Pevensey Bay
Were buried under the snoo.
All thro’ the night the blizzard fiend
Did like a lion roar
The snow rose up from inches three
To inches three foot four…
And ooooo the snoooooo…”
Is that the dreaded William McGonagall and his Tay Bridge Disaster?
I lived in Yorkshire (near Harrogate) from the ages of 12-14, and developed a spot-on accent pretty quickly – to the point where one time, when my family was vacationing elsewhere in Europe, a Brit overheard me talking and could tell we lived in Yorkshire. When we moved back to the States my mom would make me “perform” for people. It was pretty annoying. I’m just glad that I never mastered Broad Yorkshire … though think of the entertainment value…
Funnily enough, my best friend there could never quite get the hang of a “generic” American accent: it always came out sounding quite southern.
Me, too!
“Shut up, cunt. You louse. You got some fuckin’ neck ain’t you. Retired? Fuck off, you’re revolting. Look at your suntan, it’s leather, it’s like leather man, your skin. We could make a fucking suitcase out of you. Like a crocodile, fat crocodile, fat bastard. You look like fucking Idi Amin, you know what I mean? Stay here? You should be ashamed of yourself. Who do you think you are? King of the castle? Cock of the walk?”
Followed by a punch to the stomach, assuming the person hasn’t fled.