How to speak British, Course 101:

British English is considerably more complex and difficult than many writers would think. It has differed over eras of the very long history of the UK, and the writer needs to be cognizant of the historical usage of differing terms as slang and colloquialisms. The English language as spoken throughout the world is rife with diversity; there are multiple distinct accents and dialects throughout the UK, Asia, and the United States. If one attempts to speak like an individual from any of those given regions, realize that considerable personal experience and/or research will be necessary to pull it off.
The present author will attempt to provide glossaries of words and terms with care to identify the region a character comes from. As mentioned before, remember that less is more. Spoken English shows great variation across regions where it is the predominant language. The United Kingdom has a wide variety of accents, and no single British accent exists; know your history, geography, level of education, and social class. Writing narrative is almost always easier and better using Standard English, and then adding the spice and color from phonetics, pronunciations, and vocabulary, of the region.
In general, accent is the part of dialect in a local pronunciation. Secondary English speakers tend to carry over the intonation and phonetics of their mother tongue in English speech. It takes patience and due diligence to find the correct singularities of regional speech and to stay true to that manner of speaking throughout the character’s utterances in the book or story.
The following are brief comments on the variability of English:

British English
Here are a few examples—
•abercrombie–term for a know-it-all, this word from the 1930s may have just inspired a whole chain of shirtless models, booming loud music, and spray-tanned cashiers at every mall in America
•applesauce–Back in the ‘20s, the term for mocking someone who was full of baloney
•arf’arf’an’arf–British term gives us another word for being embarrassingly intoxicated.
•bev—2019, a handsome man
•bevvy–short for “beverages,” [meaning alcoholic, most often beer, and not water or milk, which will rust your pipes]
•bird—a girl, especially an attractive one
•blimey—an expression of surprise. e.g. “Blimey, what just happened?!”
•bloke: same as “dude” in American English.
•bloody—a term of emphasis, especially for the lower classes or for those who like to speak like them.
•bog—toilet. A bog roll is toilet paper. [in America, a bog is a muddy wash. The two nations are separated by a common language]
•bonkers–crazy.
•cat’s pajamas or cat’s meow—like he or she is like groovy
•chav–a young hooligan—usually of the low class–who starts fights and makes trouble
•cheeky–slightly rude or disrespectful behavior but in a charming or amusing way. A cheeky child is brash or disrespectful and headed for trouble.
•crack on—get going, get it done
•chuffed–happy or delighted.
•cock-up–A “cock-up” is a mistake or failure, and nothing to do with roosters or naughty words.
•cracking–particularly good or excellent
•cuppa—the quintessential British cup of tea [does not refer to other fluids like coffee, milk, or bevvy]
•daft—someone or something that is a bit stupid or crazy.
•dewdropper—1890s, slang for slacker
•dodgy–suspicious or unreliable.
•dog soup—Water; Dogs drink water, so offering a person a glass of water in the 1930s meant you were essentially offering them dog soup
•ducky shincracker—British for “he/she must be a good dancer.”
•faffing around–doing nothing particularly productive or taking unnecessary time to do something that should be relatively quick or straightforward. [it is a popular British pastime]
•flutterbum—1950s, a compliment on someone’s appearance. It would not be safe to do that in today’s hypersensitive atmosphere.
•foozler–affectionate term for a person who is clumsy or incompetent.
•gasper—early 1900s slang for a cigarette; later known as a “coffin nail”.
•gas pipes—a Victorian term for gentlemen with long legs who wore especially tight trousers. Somewhat akin to today’s “cool dude”.
•gigglemug—1920s, someone who cannot or will not stop smiling.
•giggle water—originally, liquor can break down inhibitions and make you giggle. But this is just a weird way of talking about alcohol. It was cool at the time
•git—a chav insult describing a person—almost always a man–who is very unpleasant, incompetent, or is an idiot
•gob–mouth
•gobsmacked–to be utterly shocked or surprised beyond belief. “Gob” is a British expression for “mouth”.
•grafting–Scottish slang for a lad who is trying to get a girl to like him. A form of flirting
•gutted–bitterly disappointed
•fiver /tenner—5 pound or 10 pound note respectively
•handcuff—1950s, engagement ring
•hip, groovy, cool—1920s and apparently forever afterwards.
•hornswoggler–1908-1920s, somebody you suspect of cheating or swindling you in some way.
•hotsy-totsy–Something was not so good, but now it’s great
•innit—a British slang corruption of “isn’t it?” Like the word “like” it is filler or sometimes genuinely for seeing confirmation. e.g. “Hey, you sod, the Taylor Swift concert is gonna be great, innit?”
•kerfuffle—a German word now frequently used in the UK. It indicates a strong disagreement, to fuss, or as a confusion [as in the original German]
•khaki wacky—US In 2018, supporting veterans means thanking them for their service. But there was a time when having an admiration for the military meant that you were really impressed by their khaki uniforms. You might even say you were wacky for it
•knackered–extremely tired
•knuckle sandwich
•lad–boys and younger men, often the younger help, or an older male so designated, as in a ship’s hand/boy/lad
•leg it–run away.
•lost the plot—to become either angry, irrational, act ridiculously
•meathook—hand, from a time when more people had hooks coming out of their arms than actual flesh-and-blood hands. It was a rough time to be alive.
•minging–disgusting or gross
•muffin-walloper–an old or unmarried woman who enjoys gossiping with her friends
•mug—cockney for a fool or stupid person. [the word in the US generally refers to a criminal/thug/or face]
•muppet–a British insult meaning an ignorant, generally clueless and ignorant
•mutton shunter–This 1883 slang term for a policeman which leaves the present author with questions and no good answers as to what it means or where it came from
•nick—to steal. e.g. Some blighter nicked me pea mash. Blimey!”
•nose bagger–someone who takes a day trip to the beach, the park, or for a picnic in the woods. He brings his own provisions and does not contribute at all to the resort that he visits.
•nosh—food. e.g., “That kidney pie is real good nosh!” [there is no accounting for taste]
•pants—underwear but also “bad”. e.g. “That’s pants!”
•pigeon-livered–Victorian-era slang for someone who behaves cowardly.
•pine overcoat—a coffin
•pint—20 fluid ounces [of beer or ale, of course], one of the essentials
•prat–slang insult term denoting someone who is full of themselves and also stupid and/or deluded
•proper—very/extremely. It is properly used in a sentence like, “That pea-mash’s proper good nosh, innit.”
•pumblechook—British-“basest of swindlers” in the Charles Dickens’ novel Great Expectations, a term that caught on in London, a city full of them.
•quid–pound sterling
•row-de-row–a play on the word “rowdy,” usually when a simple altercation turns into a full-blown fight, pronounced like rowdy
•slag off or to slag someone off–make fun of a person by verbally attacking them.
•sod—noun: devil/thing/or man, referring to a person, usually a man. e.g. “You stupid sod!” or “You lucky sod!”. verb: to shoo away, e.g. “Sod off, you stupid sod; you have lost the plot.”
•sockdolager–somebody who is particularly remarkable and special, or at least thinks they are
•throw a wobbly—an adult having a tantrum
•to take the “piss”–to mock, or generally be sarcastic towards something. It has nothing to do with being angry or urinating; hence, it is not necessarily a naughty word
•pissed—drunk [too much bevvy]; again, not a naughty
•skint—to be broke
•trollied—plastered/drunk
•unmentionables–during a time when underwear seemed scandalous, as opposed to now when everything shows, and nobody cares
•wisenheimer–a person who behaves in an irritatingly smug or arrogant fashion, typically by making clever remarks and displaying their knowledge. a person who behaves in an irritatingly smug or arrogant fashion, typically by making clever remarks and displaying their knowledge [self-approving, self-congratulatory, self-admiring, gloating] 0:35
•zorros—1950s, a type of feeling; anxious or nervous. 1950s
•zozzled–how you consumed or what you became from too many alcoholic beverages and as a result, made a fool of yourself.