I'm always using words that I forget are quintessentially British (some overlap with Australia) and people over here have no idea what I'm talking about. Like not so long ago, I said to my girlfriend that I'd quite like a biccie (cookie, short for biscuit) and she had hysterics. Now she can't stop saying it.
However for the absolute best in potty British words you have to read P.G. Wodehouse. Things like "snifter" (alcoholic beverage), "tinkerty tonk" (goodbye), "stinko" (drunk) and "nature's last word in cloth-headed guffins" (an idiot).