Chan
The gypsy word for "child" is "Chav", they used it as a term of endearment and it got picked up on and made into a racial slur for gypsy. At some point it became the English word for a certain type of working class, baseball cap wearing type.
In a world where Japan for some reason became an English Dominion, and Japanese people immigrated to England "Chan" might be used in a similar way to mean a certain type of Japanese working class sort.
Shlemiel
One of the many Yiddish words for "idiot". Yiddish is an amazing language to call someone an idiot in. There's so many different ways of saying it. I love the language and I wish English borrowed more words from it.
Mosleyan
Not really an insult, but I'm imagining it being used a bit like a cross between "Nazi" and "Orwellian" todescribe people who act in a way that is similar to Britain's short lived fascist dictatorship... On that note:
Orwellian
In a world where Orwell died before making 1984, "Keep the Aspidistra Flying" becomes his most famous book. An Orwellian life-style is one that is dirt poor, self pitying and almost entirely hopeless.
Churchill-Era
Used in papers in Britain in a Nazis win WWII universe. Means inefficient, totalitarian and (of course) evil.
Saesneg
Welsh for English. I am English and I live in Wales. So I know if someone says it they're insulting me. Though I speak very little Welsh. In a world where the Welsh played a bigger role in the colonisation of Austalia, say, I could see it becoming one of the major racial slurs against the English.
Fek!
Esperanto for excrement. Quite a nice little expletive.