Old and New Diplomacy, UK, USA and Russian Federation

The Big Three. Really?
I would like to pay the attention to US and UK relationship and changes within last five decades. I will have to mention The USSR (later Russian Federation) as it was and is in one or the other way important neighbour for both. How drastically can change the relationship between states due to the leadership or sometimes not reasonable leadership ambitions.
1941 USA, UK and USSR - allies, the Big Three. Everything is achievable and enemy will finally be defeated. The relations between the Northern allies and Soviets are tense but reasonable, thanks to enormous risks (on both sides) that leaders were willing to take for the sake of stability in the World. Can we say that W.Churchill, Franklin D. Roosevelt and J. Stalin were good diplomats? I believe we should.
Unfortunately as soon as Germany was defeated common enemy disappeared and everybody wanted a bigger slice of the "power" pie. Was UK really as strong member of the Big Three? Can we say that there were The Big One, The Big Red One and UK. UK was never equal in military power to the other two. Soon the USSR became potential enemies to the Northern Big Two (UK and USA), rather than valued allies (Constigliola 2010:2). Was the Cold War was inevitable?
Next question is complicated and requires imagination. If the Cuban missile crisis would take rather tragical turn. Soviets would attack USA, USA would attack Soviet Union and so on. Which position do you see United Kingdom taking during that period of time? Would UK be powerful enough to help their main allies or even protect itself? If USA and Soviet Union would have destroyed themselves, would UK be the new and only superpower?
During the Cold War UK and USA relationship varied and it continuous to do so now. What is really strange is that in the last two decades it is less to do with both countries still being an allies, but more with personalities. The highs of friendship and interstate cooperation could be mentioned as "Thatcher - Reagan" , "Blair - Clinton" and "Blair - Bush" coalitions. So how much can a relation between leaders affect the interstate cooperation?
Sadly but half a century on and USA and UK (and EU) are again at the very low level of cooperation with Russia. Back to the basics of secret spy assasinations and so on.
Thousands of possible outcomes of any diplomatic relations between states, but modern diplomacy has got some guidelines rather than ambitions to follow.