London was concerned that this western expansion would prove to be prohibitively expensive, a military drain on scarce resources, and not least could potenitially bring the British Empire into direct conflict with the India tribes, and the the French and Spanish Empires.
That the Americans hav convinced themselves and British that the colossal expansion of the United States across the continent of North America, wasn't both imperialist and colonialist, is rather impressive. Historical misinformation on a massive scale, and this is before we even look at the famous Monroe Doctrine.
I personally ran into this a Uni. I got into an exchange (silly me) with the head of the history department, when I questioned his refusal to characterise the conquest of the 'West' as a grand imperial project that was worth analysing in detail, as imperialism, because to me it seemed to qualify on all fronts. In those far off days, I dreamt about writing a Phd about the rise of the American Empire across the continent. Not because I had anything particular against it. Those were, after all, the days. Only I thought it was 'amusing' that everyone denied, especially British historians, what was patently obvious, that the US was an Empire and had been from its inception. In fact it's also interesting why the British chose to ignore the character of what became it's main global rival. Maybe it was because they lost virtually everthing to it, finally becoming absorbed into it. I was so young and foolish then.