Time for some more slang from the UK.
Full monty - Since the movie has come out of the same name I have heard some odd Texan descriptions of what the full monty means. It really has nothing to do with taking your clothes off. It just means the whole thing or going the whole way. That's it. Clearly when applied to stripping it means not stopping at your underwear! The origins of the expression are still under discussion. There are many theories but no conclusive evidence at the moment.
Full of beans - This means to have loads of energy. It is a polite way of saying that a child is a maniac. I was often described as being full of beans as a kid and now it is my wife's way of telling me to keep still when she is trying to get to sleep. Strangely the same expression in some parts of the US means that you are exaggerating or talking bollocks!
Her Majesty's pleasure - When visiting England, try to avoid being detained at Her Majesty's pleasure. This means being put in prison with no release date!
Hiya - Short for hi there, this is a friendly way of saying hello.
Honking - Honking is being sick or throwing up. Presumably this is a problem in New York where there are signs on the streets that say "No Honking".
Horses for courses - This is a common saying that means each to his own. What suits one person might be horrible for someone else. If my Dad was trying to understand why my brother had wanted to get his ear pierced he might say "Oh well, it's horses for courses I suppose"!
How's your father? - This is a very old term for sex which plays on our apparent British sensitivity. Rather than saying the actual "sex" word you could refer to having a bit of How's your Father, instead - nudge, nudge, wink, wink. The sort of old fashioned saying dragged up by Austin Powers.
Hump - If you have got the hump it means you are in a mood. If you are having a hump, it means you are having sex. Care is advised when you try using these words for the first time. It could be embarrassing!
Hunky-dory - My English dictionary tells me that hunky-dory means excellent. We would generally use it to mean that everything is cool and groovy, on plan, no worries and generally going well.
I'm easy - This expression means I don't care or it's all the same to me. Not to be confused with how easy it is to lure the person into bed!
Irony/sarcasm - The cornerstones of British humour. This is one of the biggest differences between the nations. The sense of humour simply doesn't translate too well.
Clancy's comment: I like a few of these. Many are quite common in Australia.