The Soviets in 1945 were exhausted.
I wouldn't be cheap and easy by any means, but a UK and US v USSR fight in the middle 40's isn't one where the Soviets have the advantage. I've seen this debate come down to 'how many Americans die before its over?' too many times to believe for a second that 'Anglo-American defeat' is the probable result.
They were exhausted, yes, so there would be morale issues and their men might not be at their very best, but they also had a considerably greater number of soldiers in Europe than the Americans, the British and the French did and they retained the ability to supply them. Especially given the morale boost of "these Anglo-Franco-American capitalist pigs we fought beside for so long have betrayed us! They're just like the fascists!" (whether or not it's true, Soviet propaganda can say so) I highly doubt it would be a question of "how many Americans die before its over?".
The western powers had lots of men, vehicles
et cetera in Europe, it's true—but not as many as the Soviets, and Anglo-Franco-American equipment and doctrine weren't so vastly superior to their Soviet counterparts to tip the balance there, and both the Soviet Union and the western powers could supply their armies. The Soviets would also have the boost of lots of communist resistance fighters in France.
I would like to say this is a very biased question considering to majority of the world, US was defeated post WW2 although in a non greatpower loss way rather than great power to great power loss.
True, but what I think the OP means is losing a full-scale industrial war.
The obvious example, one might think, is the American offensive into North Korea, which was an industrial war and yet an unqualified failure.
Going back to the question, I believe the British Empire can defeat the USA since the timeline is only upto 1899.
All the British need is to beat American navy. Yes, we know the US can challenge/irritate the British at sea. Just like the Chinese can challenge/irritate the Americans in Asia today. Sorry to say but the American Navy is not at par with the British Navy with a time limit of 1899.
Imagine if the British interferred in the Spanish American war and their fleet to support the Spanish. Imagine what happened if the Battle of Guantanamo or Battle of Santiago de Cuba was a decisive defeat which is a very high probability for the Americans.
Considering that US Navy was still into Mahan, US Navy's 7 battleships wont work vs the Royal Navy.
That's true. For this, I would suggest the Venezuela crisis rather than the Spanish-American War, because IOTL the US Army was weaker in the former than in the latter. The usual retort in Anglo-American wars—"but the USA would take Canada!"—doesn't work at a time when the US Army has only 20,000 men. In that scenario, an American surrender, "peace with honour" (without reparations or territorial losses), resumption of Anglo-American trade and recognition of British Guiana's inclusion of the disputed territories between British Guiana and Venezuela sound likelier to me than some huge, expensive American recruiting effort followed by a vast campaign northward into Canada over the issue of some South American land that most US citizens don't care much about.
But it's also true that the UK would be very unlikely to go to war with the USA over either the Venezuela crisis or the Spanish-American War, however. It's not Sealion-esque impossible but I think you'd have to change British and/or American attitues significantly to make it happen. Bluntly, the UK didn't care about Spain and neither the UK nor the USA cared very much about Venezuela.
The problem I think everyone count the British empire as only the British Isles.
I don't think that's the problem—in 1900 the power of the entire rest of the British Empire was insignificant compared to that of the British Isles. Indeed, if the United Kingdom lacked most of Canada and only ruled Prince Edward Island and Jamaica (useful naval bases near the USA), it would be in a much better position in any war against the United States because it wouldn't have to defend a vast overland border from the US Army.