I disagree about Dry Britsh wit and sarcasm being the same thing. Truly genuine british wit though is rare. Oscar Wilde, Douglas Adams and Edmund Blackadder being the finest examples.
"Like the blessed Saint Francis I'm married to poverty, unlike Saint Francis, it is not a happy marriage."
"They say harsh words can hurt more than phyisical pain. They are of course wrong, as you shall learn when I push this toasting fork through your head."
"Disease and Depravation stalk our land like..two giant stalking things."
"The majestic silvery ships hovered menacingly over the city in much the way that bricks don't."
I hardly see sarcasm as I understand it in any of these quotes..and they are all distinctly briitish, and distinctly witty.
That being said, such wit is often attributed to sources that fail to deliver.