For the US in the 1930s and into the 40s the priority would be to avoid any "foreign entanglements." Lacking any TTL WWII, there'd be no push within the US to support the UK in its stand against the Nazis as the Nazis ceased to exist shortly after Hitler's death. With Germany in disarray and its plans for European domination on hold, at the least, the USSR would no longer be getting all those wonderful high-tech German machine tools and equipment as the Germans would need a lot less raw materials from the USSR since their demand was ebbing.
In such a scenario, I still can't see the UK allying itself with the USSR. The evil monster of Nazism has been banished in Europe - Fascism being something else and not nearly as malevolent (or so perceived) - so there'd only be the evils of Soviet Communism. Those predated the rise of the NSDAP and even with Hitler's death and dissolution of that party, the evils of the Soviet Union would still exist.
The UK would also most likely oppose the Soviets on the same principle as they'd always opposed the Russians when it came to doing anything to enhance Moscow's role in Europe or elsewhere. A stronger Soviet Union / Russia is a threat to the interests of Britannia and thus something to always oppose for any good Englishman. Yes, perfidious Albion would make deals from time to time but that was all in service of The Great Game and always with an eye to keeping things uncontrolled on the Continent.
So if the USSR is economically imploding by the 1950s then the Brits would, if anything, be doing what they could to hasten that implosion rather than doing anything to keep the Soviet Bear propped up.
As to a conflict with the US, I really can't see that.
If the US has become even more "isolationist" in this ATL than in OTL, the Brits would just pretty much leave America alone. The US had its hemisphere and the UK could play at that Great Game everywhere else. There'd be no reason for the US to much worry - or even heed - what the British did in their own sphere of influence as that sphere didn't much cross into America's and our sphere was very much more concerned about the US and the US only. Yes, the US would exert economic pressure on foreign governments in response to domestic outcry. Hence the American position against Imperial Japan over its atrocities in China. This, in response to the loud and powerful "China Lobby" which had the ear of the US public. But sending American troops to China to fight the Japanese? Sending US troops to fight the Japanese directly? That's not terribly likely.
One thing about US military procurement in such an environment would be that it'd be all about being able to strike the nation's enemies as far from our shores as possible - but with as little use of ground troops as possible. We'd want a military that could protect our trade overseas but not one which would risk our getting involved in occupations overseas. Thus, the Navy would be getting the lion's share of War Department spending and then the next largest chunk would go to the Army - but specifically limited to its Air Corps and the procurement of ever larger and longer ranged bomber airplanes. The B-17s demonstrated how such aircraft could assist the Navy in defending the nation's coasts. Thus, even in the smaller economy of the US in this ATL, long range bomber aircraft would enjoy lots of development and procurement dollars.
So the US would be seeking ways to stand off from the rest of the world and be developing the means to assure that. So, American diplomacy would be toward that end and American military development would be as well. As long as no one tried violating the Monroe Doctrine's boundaries, I just can't see the US giving a damn what else happens in the world. At least not to the point of spurring official US government action. Private citizens spending their own money and lives to further this or that cause or faction? Sure. But nothing on America's part that'd require a declaration of war.
Now, one possible spin here is that with China still a morass for Japan, this might cause their economy to so stagnate or implode that Communism comes to power there. This with the covert help of the Soviets. A Red Japan could ally with the USSR for the economic benefit of both. Japanese tech could help develop Soviet Siberian oil and thus be a boon to the USSR and Japan
Something to think about, in any event...