Not slang, but useful to know:
In Britain, trousers are the equivalent of what Americans call pants. The British think of the word pants as underwear ( knickers will also be used). If you plan to do any shopping in Britain, be aware of this distinction! You don’t want to ask the sales clerk to help you find a pair of pants, and then suddenly find yourself in the underwear section.
If you’re getting your hair fixed, you may or may not want fringe (bangs) and you might want someone to plait (braid) your hair.
An elevator in Britain is called a lift. Restrooms are commonly referred to as loos, but in your travels, you may see signs for the WC (water closet). If you want to buy a ticket to get into a museum, you need to stand in the queue (line).
When driving a car in Britain, always check under the bonnet (hood) to make sure the oil doesn’t need to be topped up (filled up/replenished). Make sure you have enough wiper fluid to keep the windscreen (windshield) clean. Your luggage goes in the boot (trunk) of the car. And if you need more fuel, you want to stop at a petrol (gas) station.
If someone invites you to their flat, then you are going to visit their apartment.
When you have a conversation with anyone British, you want to end the conversation with a friendly “Cheers!” before you go on your way.
Know what you’re eating and drinking:
The British have come up with some unusual names for some rather common dishes. If you see Bangers and Mash on a menu, it’s sausage and mashed potatoes. Bubble and Squeak is a dish comprised of leftover potatoes, cabbage, and perhaps some beef. Jacket potatoes are actually baked potatoes (so called because the skin, or “jacket,” is still on). Prawns are shrimp.
Remember that biscuits in the UK are cookies. Chips are french fries. If you want potato chips, you need to ask for crisps.
You may want the food to take away (carry out).
If you order lemonade, you are likely to get some fizzy lemon drink. If you see cider and black at a pub, it is a highly alcoholic cider flavored…ahem, flavoured with black currant. Very sweet, but very delicious!
Common slang terms:
You’ll hear Brits refer to their currency as quid, much in the same way American dollars are “bucks” and Canadian money is called “loonies.”
If someone asks to borrow a fag off you, give them a cigarette.
In Britain, a kiss is called a snog. If someone is knackered, that means they are exhausted. If someone is referred to as "a minger", that means that they’re unattractive. If someone tells you to “Bugger off!,” well, it is suggested that you go away.
If you’re planning a trip to England soon, you might want to pick up the Lonely Planet British Phrasebook. It has a more extensive list of British slang and terminology, and other useful tidbits as well.
Lonely Planet British Phrasebook