This is a very interesting discussion. I believe that educated British speakers speak with great clarity, but the same can be said of educated American, Indian and antipodean English speakers. I think that the distinctive class markers of British English have a major influence on the audience, giving the upper-middle class British accent greater authority than other accents. However, we are speaking of the international documentary market, and your question implies a specialised segment of that market ('scientific'). My experience has been that the domestic US market favours US accents - we had to replace our New Zealand voiceovers with American voices to get our educational series sold in the US.
My belief is that most are made for the American market, and Americans find certain British accents sound smarter and more credible, namely an aristocratic dialect (not say Scottish or London Cockney), the Queen's English too say. Austrailian accents are used to convey rough and tumble outback cowboy themes, think Crocodile Dundee or the Crocodile Hunter. And Irish accents convey a mischievous and jocular personality. All of course based on cultural stereotypes.
But really, most Europeans complain that American English sounds like the speaker has a mouth full of marbles or bubble gum, they prefer / are taught British English and have an ear for the diction and clearer enunciation, which is also the reason why documentaries use British English speakers as well, they speak a more clearly understood English. This is probably true of India as well and all former British colonies, like say Malaysia?
Take it from a mumbler. No one in Europe can understand my American, but when I talk with a British accent, all of a sudden it is like thier ears become unplugged! :-)
Received Pronunciation - the "plummy" sound, the Queen's admittedly still somewhat nasal sound - has long been associated in people's minds as desirable, in the same way higher caste is desirable in India, or greater social standing or celebrity are desirable in the West. Almost all of us have these biases, which, for all I know, may be an integral part of ourselves, which in the right circumstances can spur us on to achieve greater objectives.
The irony is that all throughout the Sixties and Seventies the BBC was hiring Canadians because of their relatively accent-less voices.
Incidentally, as a side comment, people always wonder about the origin of the quintessentially canadian end-of-sentence comment "eh!/eh?" seems to me that it originated, in this officially bilingual land of mine, in the old french "hein?" as in "huh?"
Also the BBC, with a £380 M budget this year (out of £4.808 billion) going to £600m by 2014 (requested), happens to be one of the largest funders of popular big budget multi-part documetaries such as anything with Sir David Attenborough (60 years worth of quintessential series, and probably the reason for the British accented narrator meme), and many popular science series, etc on BBCs 4 channels and other outlets internationally.
National Geographic uses Lucy McNeil ( from Durham, England) as their spokesperson, and airs shows produced in the UK by NatGeo UK, and National Geographic International which is based in the UK as well, so lots of UK produced imports. Don't exactly know the reason for this, other than those stated above.
Necsus: European Journal of Media Studies
This essay is motivated by a desire to enliven established notions of documentary cinema. In part a response to the genre's ever-expanding field of operations, the aspiration stems above all f...
Dokumentation der Tagung "Die dunkle Seite der Medien - Ängste, Faszinationen, Unfälle" im Literatur- und kommunikationswissenschaftlichen DFG-Graduiertenkolleg „Intermedialität“ am Fachbereich Sprach-, Literatur- und Medienwissenschaften der Universität Siegen vom 30.11. bis 01.12.1999.
My contribution deals with the problems concerning a perspective of the media studies on the documentary film and proposes for the research the notion “media milieu”. This notion focuses the media practices of the documentary, which are essential for the understanding of the meaning of the documentary reference to reality. Meanwhile the vanishing o...