-A-

AAA:
   AA 
(Automobile Association) and the RAC (Royal Auto Club) are the two biggest auto clubs in the UK

Army Airborne:
    "Para's" (paratroopers)

Allowance (child's):
   Pocket Money

Apple Cider:
   Cider or Scrumpy (which is the proper 'homeade' stuff). 
The Brits don't mess around with non-alcoholic cider like the Americans do [I come from an Apple growing state]. When they say 'cider,' it's alcoholic and good, like their beer. [I've got to give them some credit!]

Amtrak:
    British Rail (although no such thing anymore since 'privatisation')
The Railways in Britain have gone downhill and are priced to astronomical levels (use those tourist rail passes where you can!)

Antenna:
    Aerial

Argument, Quarrel:
    Row (pronounced like 'how') 
('Ian and Jane had a big row last night')

Army Reserve:
    TA (Territorial Army)

Ass:
   Arse
('Ian, get off your fat arse and do some work!')

Apartment:
    Flat

Appetizer:
    Starter

Attic:
   Loft

ATM:
    'Hole in the Wall,' or Cash Point

Autopsy:
    Post-Mortem

-B-

Baptize, Christen:
    Christen
("Ian, should we invite friends and family to little Lucy's Christening, or just family?")

Baby Carriage; Buggy:
    Pram

Babysitter:
    Childminder, Babysitter

Bachelor Party:
    Stag Night

Bachelorette Party:
    Hen Night

Back Talk ("Don't you back talk me young man!":
    Speak Back
("Charles, don't speak back to your father; it's rude.")

Back-Up:
    Reverse
("Hey Gill, will you reverse the car out of the drive for me please")

Bacon:
    Streaky Bacon

Baked Potato:
    Jacket Potato

Band-Aid:
    Plaster

Baloney ('What a load of baloney'):
    Codswallop, rubbish, nonsense
('What a load of codswallop') and/or
('Don't talk such rubbish Jane')

Bangs (hair):
    Fringe

Bar:
    Pub

Barrette:
    Hair Slide

Baseball, Softball:
   Rounders  
(Baseball and Softball are virtually unheard of while 'Rounders' is a schoolyard game that is fairly similar, but not popular. Their hardball game of choice though is of course 'Cricket,' but that is quite different from baseball)

Basement:
    Cellar 
(The majority of [especially newer ones] houses don't have basements anyway, but are called cellars when they do)

Basketball:
   Netball, Basketball 
Basketball is known and played in the UK, but certainly not on every street corner or backyard as it is in the States. 'Netball' is a schoolyard game that is somewhat similar and usually played by girls.)

Bathroom, Restroom:
    Loo, Toilet

Beach ('Let's go to the Beach):
    Seaside

Beeper (for paging):
    Bleeper

Beer:
    Lager
(*Note: what the Brits refer to as 'Beer' isn't the same thing as what Americans, generally, think it is. Budweiser, Miller, Heinekin, Labatts, etc. are all styles of 'Lager.' While 'Beer,' to Brits, are the proper ale and bitter styles of liquid refreshment which are generally served at room temperature without 'fizz.')

Beer (lager) and 7-Up/Sprite mixed together:
    Shandy
This refreshing drink is the Brits' Summer drink. Even though they are tea fanatics, they do not drink Iced Tea!

Bill ($10 bill):
    Note (£20 note, etc.)

Billfold / Wallet:
    Wallet

Billion:
    Thousand Million

Boarder:
    Lodger

Bookmobile:
   Mobile Library

Box Cutter:
   Stanley knife

Bowling:
    Ten-Pin Bowling

Braid (braided hair):
   Plait

Brass Knuckles:
    Knuckle Dusters

Brush (your teeth):
    Clean 
('Don't forget to clean your teeth Jack')

Buck (as in slang for a dollar):
    Quid (slang for a pound)

Bumper Sticker:
    Car Sticker

Bunch:
    Lot
(Andy:"Hey Simon, can you go and get that stack of chairs?"
Simon: "How Many?"
Andy: "The whole lot?")    *See "guys" also.

Bus (local):
    Bus

Bus (long distance):
    Coach
The British equivalent to Greyhound and Trailways is:  National Express

Busy (phone):
    Engaged
('Karen's phone is engaged')

Butt:
    Bum, Bottom 
(Jane to Alice: 'Does my bum look big to you?')