Scep, I think your view of colonisation and the British Empire is a little simplistic. The British were not like the Vikings who conquered, killed and plundered; their interest was primarily trade ("a nation of shopkeepers"). Of course terrible things were done, often in the name of "civilising" and "educating" the "natives", but the empire wasn't all bad. Much of the "civilising" was done in the name of Christianity. Not trying to condone the whole project of course, just sayin..
Jeremy Paxman did an excellent series a few years ago on the empire, did you see it?http://www.bbc.co.uk/programmes/p00p1388/episodes/guide
Of course the profits from slavery played a part in funding the industrial revolution, but there were political and other factors too. France, for example, had as much scientific expertise as the Brits, but their political system was more restrictive (and still is); the British system was (is) more "laissez faire".
Regarding language, English is dominant because Britain and the USA have invested billions in teaching it abroad. Easy communication means easy trade, so of course it's economic.
In terms of numbers of native language speakers, English is only 3rd. Mandarin is top, followed by Spanish.