My latest completed novel (read, not written) is Richard D. Blackmore's Lorna Doone, a romance set in Exmoor, England, during the late 1600's. I have long been aware of the variety of accents contained in Britain - think Henry Higgins - but I don't believe I have ever read a novel with such untranslatable written accents as Lorna Doone. Fortunately the main characters are not given accents, but for the lesser characters, Blackmore renders their ways of speaking phonetically; and when one of the good men of Exmoor relates a fairly important (and fairly lengthy) story about highwayman Tom Faggus, I could not understand one fourth of what he was saying.
But for all that, I found it to Blackmore's credit that he went to all that trouble to accurately portray the speech of country folk in the regions covered in Lorna Doone. While many people think that a British accent means dropping one's h's in Cockney style, in reality the whole of Britain is covered by different "dialects," which can vary even between two towns in the same county. Some are less noticeable than others, but the interesting thing about watching an excessive amount of British television is that, after awhile, you begin to notice different accents among the actors. These are often masked by voice training; for example, Colin Morgan, who plays Merlin in the BBC series 'Merlin', is Irish, but hides his accent in the series so that it only slips out on rare occasions.
Another book that uses British dialects, but less overwhelmingly, is Burnett's The Secret Garden. Taking place in Yorkshire, Burnett brings in the Broad Yorkshire speech of the working class, which has the two-pronged effect of grounding the reader in the location and portraying the different levels of society. These are even more clearly shown in the 2004 British TV serial "North & South," based on Elizabeth Gaskell's novel North and South. Whether it was intentional or not, I found it interesting that the change from the rural, slow-moving, peaceful South to the fast and industrial North is reflected in the different ways of speaking between the Hales (who have moved up from the South) and the Thorntons (a very Northern family). Daniela Denby-Ashe, who plays main character Margaret Hale, has a very soft and "round" voice, whereas Richard Armitage, who plays Mr. Thornton, speaks through his nose and with his jaw set, giving him a harsh tone. The series also differentiates between the workers and the masters of the cotton mills, for among the workers, the dialect of the region is much more pronounced than among the higher-class "masters."
Accents must be used with more caution in writing than in film, since phonetically-rendered speech can be cluttered and confusing, rendering the dialogue unintelligible. However, they are exceedingly useful and interesting when done well and deserve at least a passing acquaintance, as some knowledge of the dialects of different regions, whether British or not, bring depth and accuracy to writing.