Writing dialogue for your British characters takes much more than just adding "mate" at the end of every sentence. While some of your American readers might not notice the error of your portrayal, it's always a good idea to be as true to your character as possible and not alienate any readers from across the Pond. With some research and attention to detail, you can create interesting and authentic dialogue that anyone will appreciate, regardless of their origin. Read on to learn how to write dialogue for fictional characters with a British accent.
Decide exactly where your character is from. You may be writing a historical novel or other work where the place is already fixed, in which case you already have the answer. If you're unsure of the location and a specific area is not necessary for your plot, read on for some ideas on how to choose the best location for your character.
Research. The saying goes that Britain and the U.S. are two countries divided by a common language. There are plenty of excellent books and websites devoted to just these differences, that are immensely helpful for creating realistic British characters. You can find some great slang phrases to add color to your dialogue, but it's also important to pick up common words that are jarring to British readers if you get them wrong. For instance, they might say "jumper" instead of "sweater," "trainers" instead of "sneakers," and "car park" instead of "parking lot" (see Resources).
Visit British websites and/or read British publications. As with any language, perusing a dictionary or list is fine up to a point, but seeing the language in use is vital to understanding it. Going to a British magazine or newspaper website can help you see the language used by natives and read or view interviews to get a realistic sampling of dialogue.
Watch British films and television. This is particularly handy if you can find a film centered in the area you want to write about. "The Full Monty" for example, is a movie set in Sheffield, which is in the north of England. "Gosford Park" is filled with characters from all class levels and regions, and is also a period piece that can help you if your story is set in that era. The more you hear the language, the easier it is to mimic it in your writing.
Choose an actor. Films are created by writers, just like you. Actors are real people whose accents reflect where they're from. While some have trained to eliminate their native sound, for the most part you can have a great live inspiration for your character by studying an actor from that region. This is where all those DVD extras come in handy, on series films like the "Lord of the Rings" and "Harry Potter". Watching the interview segments on these films you can find actors who hail from Manchester, Canterbury, Sheffield, Blackpool, and London--as well as Glasgow in Scotland and Belfast in Northern Ireland.
Immerse yourself. Simply consulting a guidebook or web page to create an authentic character is similar to using one of those web translation tools to write a love letter in French. It will come out stilted and artificial. The more you read and listen to true British accents, the more you will be able to think in that way, which is the best way to write a character. When you can hear their voice clearly in your head, it will pour naturally out onto the page.
Talk to an expert. If you visit social networking, journaling, blogging or fan board sites, chances are you've run across British and European web users. Find one of these users who's willing to take a peek at your manuscript and point out any errors they might see. A fellow author might offer to edit your British English if you edit their character's American English. This is how you get invaluable advice about grammar, like the use of "got" versus "gotten" and notice of small details, like "zip" instead of "zipper". These readers can help make your character be as authentic as possible.
Don't overdo spelling out the accent. Some authors will definitely disagree, and a linguistically heavy historical novel might have some call for being as rigidly accurate as possible. In most cases, however, a little goes a long way. Focus on getting the vocabulary right, and the phrasing. Dialogue riddled with apostrophes, slang, and alternate spellings can end up making your text annoying at best and unreadable at the worst. So, go with: "You been lookin' for him all week. I were about to send out a search party!" rather than: "Ya behn lookin' fur 'im all week. I were 'bout ta send't a search party!"
Remember that people are individuals, no matter where they're from. Think about where you live and what someone might think of as your region's accent. It might not be the way you talk at all. The character you write is an individual as well, who can have their own differences and quirks. Don't worry about obsessing over the dialogue to the point of making them into a textbook example. Get the right flavor of British English and focus on your character's personality and story--your readers will believe in that character wholeheartedly.