I found a series of short articles by Arfken and Leggett in BNA Topics on mail to England at this time period. (See Vol 51 #2 Whole #459 (Apr-Jun 1994) starting on page 20.) You can find a pdf version on the site below:
https://bnaps.org/hhl/hhl-index.phpA quick summary of what I saw there.
As of 1857, the British weights were indeed ½, 1, 2, 3, ounces (as chipg stated). However, this was changed as of January 1, 1866 to by the half ounce, so a 1-1½ ounce cover would be a triple rate.
At the post office's discretion, a letter intended for, say, the Allan line could be switched to the Cunard line (as surmised by John Becker). This would generate a more to pay in England. To determine if this happened, you need to determine which line carried the letter. To do this you need accurate dates and shipping tables.
Lastly, the Cunard line went via New York (so the extra cost is the US portion), while the Allan line went via Quebec in the summer, and Portland Me. in the winter (because of ice in the St. Lawrence).
The articles also give some further references.
Jan