From 1776 til this very day, the upper class in Great Britain has disliked the United States. The Tories and the socialists despise us equally but for opposite reasons. Don't confuse them with the ordinary bloke in England.
Gladstone was no different that a lot of people back then, or today for that matter. They dislike people as a group, in this case Americans, but had individuals as friends. In Gladstone's case it was people like Charles Sumner.