Here comes some typical British expressions that I've come across while writing Harry Potter fanfiction.

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Expressions
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•Go boil your head = Buzz off; go away.
•Having kittens = Stressing out.
•Lie in = Sleeping in.
•Manky = Bad, inferior, defective, dirty.
•Nutter = Crazy person.
•Outta your tree = out of your mind.
•Peckish = Hungry
•Rabbiting = Gossip idly.
•Swot = someone that studies hard, Hermione or Percy is swot's for example.