How do you respond to a dinner invitation that says "Eight for eight thirty"? What might induce you to get off a London train at a place called Mud Chute? When is it okay to drive over a sleeping policeman? And why do teh Brits keep saying "Who's she, the cat's mother"?   Rules, Britannia is an invaluable resource for Americans who want to make a smooth transition when visiting or relocating to the UK. This entertaining and practical insider's guide contains scores of established do's and dont's that only a Brit would know.   Most of us know that an elevator is called a "lifet," a toilet is a "loo," and the trunk of your car is the "boot," but who would have a clue about a "sprog" or a "gobsmacked berk"? These phrases are part of daily conservation in the UK, and leave many visiting Americans as baffled as if they listening to a foreign language.   Covering such essential topics as vocabulary, house- or "flat"-hunting, business culture, child rearing, and even relationship etiqutte, Rules, Britannia will ease the anxiety that comes with a transatlantic move or extended visit, and is sure to make any old Yank feel like a regular Joe Bloggs.