Strange character accent choices in The Game of Thrones

I don't often frequent the media and entertainment website Mashable, but I was quite interested in an article on the various English accents in the TV adaptation of The Game of Thrones (I say that advisedly, lest we forget that there was actually a series of books too at one time).
The most prominent accent in the series as a whole is the gritty, down-to-earth, northern tones of Yorkshire. This is the accent employed by Ned and Robb Stark and Jon Snow (perhaps as close to the "good guys" as The Game of Thrones gets), by Robert Baratheon, Theon Greyjoy, Bronn, and pretty much all the northern lords, as well as Mance Rayder, Ygritte and the other wildlings. However, although Sean Bean (who plays Ned) and Mark Addy (who plays Robert) are native Yorkshiremen, most of the others sporting the accent are not: Richard Madden (who plays Robb) is Scottish, Alfie Allen (who plays Theon) is a Londoner, Kit Harrington (who plays Jon) is a posh Londoner, Jerome Flynn (who plays Bronn) is from the southern county of Kent, and Rose Leslie (who plays Ygritte) is Scottish but speaks naturally with a posh English accent. It's a credit to the series' actors and language coaches that I, who was brought up about 10 miles from the Yorkshire border, had no idea that they were not "real" Yorkshire dudes. Indeed, I remember being impressed that they had found so many native Yorkshire speakers for the series!
The "posh" English accent - otherwise known as "received pronunciation", "BBC English", or "the Queen's English" - is mainly reserved for the arrogant and aristocratic Lannisters (Cersei, Joffrey and Tywin) and the equally aristocratic Daenerys and Viserys Targaryen, but also Brienne of Tarth, Gendry, Varys, and, perhaps strangely, the Stark women, Catelyn, Sansa and Arya, as well as the younger, less-macho Stark boys, Bran and Rickon, all of whom clearly take after Catelyn's social-climbing side of the family, not Ned's. There is perhaps a trace of northern-ness to the Starks' posh, as opposed to unadulterelated posh of the Lannisters: Michelle Fairley (who plays Catelyn) is actually Irish, Maisie Williams (who plays Arya) is originally from the West Country, and Sophie Turner (who plays Sansa) is a Midlands girl.
Interestingly, other than a bunch of soldiers and guards who serve merely as cannon-fodder in the series, the only Cockney among the main(-ish) characters is Stannis Baratheon, although I must confess I don't remember it being that obvious. Stephen Dillane (who plays Stannis) naturally has a very posh English accent, so why someone decided he should add in a Cockney twang is a mystery. Likewise, the only Geordie in the series is Ser Davos Seaworth, who is played by Liam Cunningham, an Irishman (although I have to say he does a very convincing job with the Geordie accent). Were these casting decisions, directorial decisions, or just no decision at all (random)?
Several characters, mainly those from Essos and the more distant, "foreign", regions of Westeros like Dorne, have what might be described as "vaguely Mediterranean" accents, perhaps designed to indicate a generalized "foreignness". This includes Melisandre the Red Lady (who is actually played by Dutch actor Carice van Houten), and Syrio Forel (played by British-born Greek-Cypriot Miltos Yerolemou).
The only outliers among the Lannisters, both in terms of accent and character, are Jaime and Tyrion. Jaime is played by Nikolaj Coster-Waldau, who hails from Denmark. Like many Danes, he has a ridiculously good English accent, but it is noticeably not a Lannister accent, and that may be deliberate, to separate him a little from the stain of malice and evil that marks most Lannisters. Tyrion is played by Peter Dinklage, who gets all the best dwarf parts these days, and does a pretty good job with them, it must be said. Dinklage is American, though, and his super-aristocratic accent comes over as almost a parody and, as the Mashable writer points out, prone to wander a little.
The other actor with a wandering accent is Irishman Aidan Gillen, who plays the devious Lord Petyr "Littlefinger" Baelish, whose on-screen accent can be anywhere from Cockney to Welsh to mild Irish, depending on the scene. Why was he instructed to speak with anything other than his natural Irish lilt, which would have suited his character perfectly?
So, there are definitely some interesting casting and directorial choices. But, when push comes to shove, I'm not sure that the characters' accents are that important to the series. The mish-mash of different strong accents is probably more confusing to North Americans than it is to Brits, who are more used to coming across different and/or displaced regional accents. I have a suspicion that many North Americans are probably looking for deep meaning and significance in the accents used, meaning that is not even there.