Why are so many video presentations and tutorials on the web.......
 

narrated by a guy with a British accent?

It's a conspiracy.

William F. Buckleyesque accents? :)

no, he had an affected Ivy League accent - these are real British accents, not Cockney, the snooty educated type Brit accent. I'm sure it's done because they speak well, clearly enunciate their words. British narrators are used fairly frequently in American advertising - usually to give whatever's being advertised an air of sophistication.

Actually, Buckley's accent is genuine. Here's the info http://www.slate.com/id/2185368/