The British were big and the main force. The American colonies occupied such lands, the British claimed. Falsely, British colonists disagree with the claim of American colonialism. Throughout history, the British have experienced many successes and failures. Most of the times, they only search for land to be lost in another country. For example, when India decided to declare independence, the efforts of the British to conquer India were in vain. Similarly, the American colonies were separated from the British. So no, I don't think the British thought that the abuse of the colonies was in any way unfair.
Mexican Cession. Mexican Cession lands were captured in the Mexican–American War in 1846–48, and ceded by Mexico in the Treaty of Guadalupe Hidalgo, where Mexico agreed to the present Mexico–United States border except for the later Gadsden Purchase.
