“i Strongly Disagree!!”
Call them Scottish. That will raise hackles wherever they are, for various reasons. 
So I guess “Piss of, ya f***in’ Limey!” might be considered a bit…half-hearted?
“nice teeth”
throw out some rhyming cockney. like call someone ‘sherman’ for wanker. expect to be called a ‘septic’ back.
calling someone a ‘wally’ is pretty mild like dork.
sheep shagger works on the more rural gamers
bastard seems to be pretty universal
You’re the fucking problem you fucking Dr White honkin’ jam-rag fucking spunk-bubble! Or anything else Ben Kingsley says in Sexy Beast.
Toe-rag
Calling a dispatched fool your bitch works regardless of the dialect.
On the other hand, if you want to be nice* to people, you might call them “safe” (as in, “He’s safe, he is” = he’s an alright guy).
*Yes, I know, nice! On the Internet! In an online game! Ha! But it might come up. 
Call each of 'em a big girl’s blouse (wimp). Unless they are in fact girls, in which case call each of 'em a daft bint (idiot).
Or just call 'em a lot of spotty-faced gits.
Here is a site that will help Americans decipher British English…and they even mention the word “bespoke” which I only heard for the first time recently.
But quite a few other words and phrases are listed that should have you insulting them damn furners like a pro in no time…of course, there is always the danger you might besmirch America’s reputation as a warm and fuzzy nation, but hey - as they currently say in Washington DC, fuck 'em if they can’t take a joke.
Not sure that these are all strictly Britishisms, but try some of these:
“Don’t throw the toys out of the cot” (don’t get upset)
“No need to spit the dummy” [dummy = pacifier] (ditto)
“Who rattled your cage?” (when someone comes in to a conversation who perhaps doesn’t usually speak up much)
“No Old Baileys” (even a lot of Brits won’t get this, but I learned it from South Londoners when I was doing missionary work there - just kidding) (means “Don’t moan about the result/your bad luck, etc” (the Old Bailey is the building that houses some famous court rooms)
God, how this dates me - to me, it will always be Miles Hunt for tyhe rhyme.
I thought it was “merchant banker” – or perhaps merchant bankers are in fact all wankers.
I think merchant banker is the default option. The trouble with rhyming slang is twofold: first, authentically, you should only give the first, non-rhyming part (as in “china” [full form “china plate”] = mate), otherwise it’s like having a 4-digit code but telling everyone the first 3 digits; second, there are quite a few variations, so that some people just won’t get it. For example, in the days before I joined the dope, when I was less PC than I am now, I used to say “iron” [“iron hoof”] for “poof”. Those among whom I worked as a missionary in South London understood it - they would, they taught me - but others might not get it at all, preferring their own rhyme, or indeed eschewing entirely all rhymes in this lexical field as a matter of principle.
See, this is just offensive, and with the ethnic make-up of much of South London comes across as pretty racist.
Smeghead!
You might like to try ‘knobstick’ and ‘shagsack’. Neither of them have particularly literal meanings, but they generally work.
However, I think generally speaking, using unfamiliar slang (or from a foreign culture) usually ends in embarrassing failure for the speaker.
Anything involving the implication that someone lives in a Caravan is pretty insulting, although it does have some slightly unpleasant racist undertones (against Gypsies, also known as Pikies), so it’s probably one to best avoid…
Heh. Y’know, GM, I once climbed over the wall at Portman Road. But a policeman saw me…
and made me go back in and watch the rest of the match.
Go back to your council flat ya chav! 