British-English Dictionary

With National Novel-Writing Month (NaNoWriMo)* fast approaching, I have made a potentially dangerous decision about my next work-in-progress. I am going to set it in England.
Which means that, dun dun dunnn... My characters will all have to speak Brit.
"Surely Brit-speak can't be that different from American," you say. "After all, it's the same language!" Well, first of all that depends on how you define language as opposed to dialect.** But okay, let's assume they're just two different dialects. How different can they be?
Most people already know about the funny spelling (what's with all of the ou's and ae's anyway? Not to mention their aversion to the letter z, pronounced "zed" in most British dialects) and the general accent. Maybe you already know some of the more common alternate words, too: "arse" instead of ass, "flat" for an apartment, "mobile" for a cell phone, "trousers" for pants and "pants" for underwear, etc.
But there are some really weird alternate words that no one thought to warn me about in advance. For example, apparently an exit ramp over here is a "sliproad." It took me ages to figure that one out ("Ellen, go to the bus stop on the sliproad." "But it's raining. All the roads are slippy?"). And an ATM is referred to as either a "cash point" or a "hole in the wall." Luckily, my brother is an architecture student, so I already knew about the whole "wash closet" = toilet thing (alternately known as a "loo"), but if I were the kind of tourist to carry a fanny pack, I could have gotten into an awkward situation. As one cab driver took delight in explaining to me, a fanny pack is called a "bum bag" over here, because in England, "fanny" = ladies' private bits.
Oh, the things that tour guides forget to warn you about.
This is why I love English, though. There's so much fun word-playing to be had - especially in Englandia, where it seems like everything has at least 2-3 different names (their swear-words are far more plentiful and fun to shout than ours, too).
Anyone else have some Brit-speak to share?

*For those of you who have never done NaNo before, it's a contest where you have the entire month of November to write a novel - in this case, "novel" is defined as 50,000 words. If you win, you are awesome. If you lose, well, at least you tried?

**The short answer, by the way: "A language is a dialect with an army and a navy." But if we judge by that, then since America and Britain have their own armies and navies, Brit-speak and American are two different languages. However, if we go by the mutual intelligibility rule (if I understand you and you understand me then we speak the same language), Brit and American are just two dialects of English. For simplicity's sake, I went with the latter argument in this post