One of the things I loved when I started studying English was learning idioms. I would seek out idioms and learn them by heart. I was told that one good measure of someone’s command of a language is that person’s ability to use and understand informal expressions that native speakers use. I learnt the language methodically.
As a result, my English was a lot better than other students’ in school. 
After my high school, I went to the British Council English language classes, followed by more classes in the American Embassy for good measure. I enjoyed and learnt a lot in all those classes. I knew there’s a bit of difference between British and American English. But they all sounded Greek … (er, English) to me.
With a good command of English, I was confident of my ability to communicate when I landed in the UK 10 years ago. As expected, I was able to communicate without much difficulty. But I came across a lot of phrases and expressions I didn’t know before. Since I love learning things like that, I have had a field day gobbling up every new expression I’ve found. (See, I’ve just used one! A field day.)
I’m compiling a list of what I believe are uniquely British expressions that I’ve heard people use and increasingly hear myself use.
“It’s chucking it down!” 
As far as I knew, “chuck” means “throw away.” But people here use it when it’s raining heavily as if somebody is pouring water in bucket loads. I could have used it back in Burma when I was studying in the British Council. Imagine what my British teachers would have made of it if I had uttered “it’s chucking it down!” looking out of the window in the Monsoon season. Especially if I pronounced it the Derbyshire way, where “chuck” rhymes with “book”!
“It’s better than a poke in the eye.” I found that a bit funny. I heard my adopted dad use it often: “I’ll get £100 for that. It’s not much, but it’s better than a poke in the eye.”
Something else he says whenever I pile my plate high with food and struggle to finish it near the end: “your eyes are bigger than your belly.”
I also found out that another thing to call a man is “a bloke.” And there is also “chap.” Sometimes people greet other by saying “hi chaps” instead of “hi guys.” By now, I have figured out that when some people address me as “guys” when I’m in a group, it’s not because they mistake me for a bloke.
One expression my adopted Grandad used to use was “where there is nowt, you can’t expect owt” when he was insinuating that somebody is brainless. (Some people here pronounce “nought” to rhyme with “out”, others rhyme it with “oat”. Still others use a diphthong never used in standard English, so I can’t find a way to write it down. So you’ll just have to make a trip to Derbyshire and listen for yourself!)
“There’s more than one way to skin a cat.” I don’t think the feline resident next door would appreciate that somehow.
“There’s not enough room to swing a cat.” I am told that the “cat” there is nothing to do with the furry kind. It comes from naval jargon, from the time when people were disciplined with a vicious whip, called a “cat o’ nine tails”. Apparently, it had nine whiplashes, each one with a lead pellet woven into the end of it, so that it would hurt a lot more when the boss was dishing out the whipping. (I think that must also be the origin of the idea that a “cat” has 9 lives!)
“No use crying over spilt milk.” Especially if it’s lovely creamy Jersey milk!
“Blind as a bat.” I use that quite often whenever I have to look for something my adopted dad’s been hunting for for days and I find it straightaway.
“Deaf as a post.” (“What? I can’t hear you.”)
Here’s an expression that is not very polite but I hear it used sometimes: “dead from the neck up.” Guess which member of my English family I learned that from!
Here are a couple of not very classy expressions: “flat as a fart,” and “fussy little fart”
The tyres on my bike were completely flat. So, my Dad said, “they’re flat as a fart.” I had to snigger at that. It’s a variation on “flat as a pancake”.
When my Dad was teaching in an international school (I won’t say where), my Grandma and Grandad went to stay with him over Christmas. After meeting the headmaster, who thought himself very important, wearing a tartan bow tie in temperatures hot enough to melt the road, my Grandad called him a “fussy little fart.” Luckily, the headmaster was out of earshot.
“Some fell on stony ground.” It took me a few tries to get that expression right. I believe it’s from a well known Biblical story of a sower, who was spreading seeds, some of which missed the fertile soil.
“That is as subtle as a brick through a window.” (One of my personal favourites.)
“It’s as useful as a chocolate teapot.”
“He’s lost the plot. Awww, bless.” Not very nice, eh? 
“He’s a bit dodgy.” I understood its meaning. But when I heard “dodgy knees,” “dodgy camera,” I had to be quick working out what it meant, because “dodgy” means “suspicious”, doesn’t it? “Suspicious knees?” Huh?
“That kid is a handful.” Before I fully grasped what it means, I once used it about somebody’s kids. I said, “They are a handful.” I meant “energetic”. When the father said, “Not really, they are OK, they can be quiet,” I realized I may have “got the wrong end of the stick” with the meaning.
One word I sometimes use these days instead of “thank you” is “cheers.” When I first heard it I wondered why people were toasting me all the time. Up to that point, “cheers” for me was when you drink to somebody’s good luck or when you are cheering on a football team.
I also have learnt to use “Sugar!” instead of “Shit!”, which I was using liberally and quite happily before.
One thing I have learnt to use when I’m annoyed and when nobody else seems to be around is “For F*** Sake!” And then I hear a voice shout, “Wash your mouth out with soap and water!” (Big ears!)
After ten years of speaking English 24/7 except when I phone my parents back in Burma, I now think in English most of the time. I’m proud that I can understand everyone I’ve come across even when they are speaking in a thick Derbyshire accent. But, occasionally, my pronunciation has a hiccup. Last week, I told a friend that she had left her flea. It turned out that it was a fleece that she left, not a flea. It seems that I had “lost the plot” a bit there.
Anyway, the list is “not in any way, shape or form” complete. If you can think of any other expressions that I haven’t mentioned or might not have come across, please help me by leaving a comment below. I would love to add them to my ever-expanding repertoire.