"bat in the cave"

I've been wanting to try using British English language to write, but so far everytime I go to write I don't use any words that are spelled differently. I started reading a book of essays on all the oddities of the UK. A few pages were dedicated to rambling on using phrases that an American would have no understanding of...it worked, I was clueless. (Can anyone tell me what a counterpane is? If I already told you it doesn't count, and it doesn't count if you look it up online) So instead of just messing around with spelling, I started researching on the internet the differences between American and British English usage. There is definitely a lot of information out there. Wikipedia has lots of interesting articles although they are a little more technical than what I'm interested in at this point. There are also lots of interesting dictionaries that define lots of British words. I'll give you 10 bonus points if you can tell me what the title of this post means. Don't cheat though, nobody likes a cheater.

Apparently they also have some sort of rhyming slang that I've yet to understand called cockney rhyming slang. I found a site that said, "There are lots of words that make up cockney rhyming slang. These are basically rhyming words like "butchers hook" which means "look". If you are in London and you hear someone talk about a Septic they are probably talking about you - because it's short for "Septic tank" which equals "yank", which is our word for an American. How do you like that!"

Can someone please explain this to me??