Ace — A synonym for awesome, ace is also a British slang word to describe someone who's good at something. "Andy Murray is ace at tennis."
Arse — An arse can refer to a person's literal butt, or can be used to describe a stupid person who behaves in an annoying way.
Bloke — This one is prevalent in Britain, as well as New Zealand and Australia. It's a simple alternative way to refer to a man, dude or guy.
Bollocks — Used frequently in pop culture, but best not around grandmother as it is another way to refer to testicles. A "load of bollocks," means that something isn't true, and to "talk bollocks" is another way of saying "to lie."
Brilliant — Here's one that gets used all the time by the Brits to describe something that is wonderful. Think of it as the British equivalent of America's obsession with "awesome" or "cool."
Bugger all — British slang term for nothing, as in "I got bugger all done at work this week," or "There's bugger all in the pantry to eat."
Cheeky — If someone is described as being cheeky, that means that their dry sense of humor is showing. Also known as being a smartass, or someone who's not taking something seriously.
Cheerio — This does not exclusively refer to cereal in the U.K.; instead, it means good-bye.
Chuffed — This British slang word expresses delight at something. For example, "The fans were chuffed that England made it so far in the recent World Cup."
Dead — Another way of saying "very." For example, "This meat pie is dead delicious."
Dodgy — Remember how Hugh Grant went searching around the "dodgy end" of a London neighborhood to find his love interest in "Love Actually"? The word describes a place, person or experience that's a bit sketchy, even dangerous.
Eejit — A play on how the Irish say the word idiot. Same meaning, though.
Ends — A British slang word for the part of town or neighborhood that you're from.