I'm sure this information is already in one of the other threads, and there is really no mystery about this so I'm not sure why people keep insisting on inventing imaginary backgrounds. However, here goes again.

The Olney Hymns, which includes "Amazing Grace" was published by John Newton and William Cowper, in three volumes, on February 15th 1779. A facsimile e-text is available here. "Amazing Grace is hymn no. 41 of volume 1, based on 1 Chronicles 17 v16,17 with the text "Faith's review and expectation". No tunes are given for these hymns and, at the time, they would have been expected to have been sung to common tunes already known to the congregation. Until the publication of hymnals with tunes in the mid 19th century there would not have been a "standard" tune for all but a few hymns, each parish probably having their own (as for "While Shepherds Watch" which had over 1,000 known tunes).

John Newton was English, both born and died in London.

The tune "New Britain" (called "Amazing Grace" in Britain) first appeared in "Virginia Harmony", by James P. Carrell and David S. Clayton (Winchester, Virginia: 1831). It was adapted by the authors from an early American folk melody entitled "Loving Lambs". This tune is said to be related to the Scottish folk song "Loch Lomond" (though I have some trouble with this) and also to the 17th century English song "Toddlin' Home" (which appears as in Dvorak's "New World Symphony" as "Going Home"). Again, I have a little difficulty with the latter, though it's much closer than "Loch Lomond".

:-)