Smart People Speak the Queen's English

In American media, especially cartoons, characters who are supposed to be intelligent often speak Received Pronunciation, even if they aren't explicitly supposed to be from England. This association of RP with smart people probably comes from the days where this was the accent associated with those who were wealthy enough to receive an education at all, and the more general idea that characters who speak RP are authority figures. Upperclass Twit would be a subversion or inversion of this trope, especially if the twit is English, as in the trope namer.

This doesn't apply to brainy RP-speaking characters who are in a setting where everyone is English unless it's obvious that only the smart characters speak with this accent and the others all have other English accents (ex. Cockney, Oop North, etc). The examples are in two groups, with one being characters who actually ARE stated to be from England within the story and the other being characters who are not yet still have the Received Pronunciation accent.

Villainous examples are likely to be Wicked Cultured. However, while this can overlap with Evil Brit, it doesn't have to, as many examples include heroic smart characters. Compare and contrast Fake Brit, (that's where the actor playing a British character isn't actually British) and I Am Very British (where Received Pronunciation is the only English accent in American media, and the characters in that trope don't necessarily have to be smart).

Literature

 * Mentioned by Dave Barry, where he says a person with a British accent could be presenting Hawaii Five O and Americans would think them extremely enlightening.

Live Action TV

 * Rupert Giles from Buffy the Vampire Slayer.
 * Curiously, when a spell makes him behave like a surly teen, he speaks in a working class accent. In an earlier episode, he relates how, as a youth, he rebelled against his upbringing and fell in with 'the worst crowd that would have me' - so which accent was the put-on is up to debate.
 * Doctor Bashir from Star Trek Deep Space Nine. Mostly notable in the episode where we meet his parents (at the same time as revealing his intelligence was augmented), who do not share his flavour of British accent at all.

Western Animation

 * T.N. Honey from Strawberry Shortcake (she basically speaks only in stereotypical English phrases)

Anime and Manga

 * Arguably, the dub version of Sailor Mercury from Sailor Moon is given an accent that is at least slightly English. Luna also would fit in the dub since she's an intelligent Mentor Mascot who does a lot of strategy planning for the "Scouts" and is generally portrayed as more intelligent than Artemis.

Film (Animated)

 * Scar from The Lion King (also doubles as Evil Brit)
 * Scooby Doo in Monsters Unleashed
 * Dr. Cockroach, Ph.D. in Monsters vs. Aliens.

Film (Live Action)

 * The Brain Gremlin in Gremlins 2.
 * The X Men franchise is a mixed example. Professor Charles Xavier either speaks with an RP (first three movies) or is an actual Brit (X Men First Class).
 * Patrick Stewart is also an actual Brit.

Literature

 * In the Warrior Cats series, the last three audiobooks in the The New Prophecy series are read by Nanette Savard, an American actress. The narration and most of the characters are read with an American accent - except, for whatever reason, the medicine cats, who are read with a British accent. They're regular Clan cats, born and lived with their Clanmates all their lives, and just chose a different job - so where did the accent come from? Are they born with it and for some reason all cats with this accent take the medicine cat's job? Or does healing cats suddenly give you a different accent somehow?

Live Action TV

 * Notable subversion/playing around of this trope is Charlize Theron's character from Arrested Development. Her English accent is (according to the narrator) the reason that people don't figure out that she's . Of course, Theron isn't even English (she's South African, but is a naturalized American), which is of course lampshaded in the show.
 * Gaius Baltar speaks with an RP accent. Later in the series, it's revealed that he changed it from his family's native accent (which is portrayed as a rural, working-class English accent) to fit in on Caprica; what pushes it into this trope is that Caprican citizens are otherwise represented as varying kinds of North American.

Video Games

 * In Xenoblade Chronicles, the Higher Tech Species High Entia all speak the queen's English, in contrast to the main characters' middle- and working class accents that are typical of Homs. Reyn even lampshades it upon first meeting one, remarking on her 'posh accent' (how he would even know it's a posh accent when none of his own species speaks it is another matter).

Western Animation

 * Mr. Longface Caterpillar from Strawberry Shortcake
 * The Bookworm from the Huggabunch
 * Actual Factual from The Berenstain Bears
 * Mr. Chips the computer from School House Rock
 * In The Adventures of Sonic the Hedgehog, when Grounder gets a genius chip, his usual Simpleton Voice changes to Received Pronunciation.
 * Wile E. Coyote from Looney Tunes, in the cartoons where he has a voice.
 * Arguably, Stewie from Family Guy. There's some controversy as to whether his accent is supposed to be British or Bostonian Brahmin (eg, Charles Emerson Winchester III from MASH, Thurston Howell III from Gilligans Island.)
 * Puzzlemint from My Little Pony (the G3 series)