I had the pleasure of meeting someone, an American, yesterday who likes studying languages and had twenty years of editorial work experience (mostly in-house for a company but also some dissertations) and she told me that even Oxford University Press outsources editorial services and that the quality of their academic trade books is not what it used to be.
She said that the differences between British English, American English, and Indian English would suggest that writers writing in American English or British English should not consider using editorial services in India but look for services within one's linguistic community. So I looked for information on the Internet and found a few relevant articles.
Knowing this, I would prefer to rely on my own strength to edit my writings but this means letting the final draft sit for a while (anywhere from a few days to a few months) so that I can look at it with a fresh eye. Peer critique is very helpful for generating comments for improving content and style and professional advice (such as from teachers who are writers) is great when you can get it but sometimes it's difficult to know to whom one can turn to entrust the final editing. If I were to turn to professional editorial services, I would want to see the quality of their work beforehand. http://www.the-efa.org/