I do agree about the sell-out, tho'. I think both UK and US just allowed themselves to be totally shafted by Uncle Joe.
Was this really a sell out by UK and USA?
It should be taken into consideration that the Polish government-in-exile did not really help thier country as well as the could have especially after Sikorski's death. Sikorski was the only person with the gravitas to present Poland's views to the Allies. After Sikorski's death the disunity and infighting between the various political factions within the Polish government-in-exile that Sikorski had tenously held together just made Poland's situation even worse--the other allies had lost faith in Polish government-in-exile's ability to reach a common and united front about anything let alone reaching a post-war accord with the USSR.
However, Sikorski actually published his own proposal for a Polish post-war border settlement with USSR very much along the lines that that the 'Big Three' eventually agreed upon! So had Sikorski lived, post-war Polish borders would closely resemble that of today even without the 'Big Three'. The other allies knew about Sikorki's vision for post-war Poland it was not a secret from them!
Now if there was a reason to get Sikorski out of the picture I personally believe it would have come from right-wing elements in Sikorski government who were totally opposed to any dealings with USSR especially over border issues. Given that Sikorski was thinking of giving up Polish territory in the East in exchange for new ones in the West it is more likely that any threat to his life would have come from these elements rather than from Britain or USSR, in my opinion.
Also, had Sikorski's desire that all of the Polish forces in USSR (released after the amnesty) should fight alongside the Red Army in the liberation of Poland from the eastern front been realised then the political situation on the ground in Poland might have been different than it turned out. However General Anders opposed Sikorski's plans and ignored an order by Sikorski on the limit of Polish forces that would be allowed to leave USSR. Anders was reprimanded for this disobedience and reminded by Sikorski that he was commander-in-chief and not Anders, but the deed was done and we will never know if Sikorski's plan was the better one.