From the Wikipedia:
The Soviet aviation minister Mikhail Khrunichev and aircraft designer A. S. Yakovlev suggested to Premier Joseph Stalin that the USSR buy the conservative but fully developed Nene engines from Rolls-Royce (having been alerted to the fact that the U.K. Labour government wanted to improve post-war UK-Russia foreign relations) for the purpose of copying them in a minimum of time. Stalin is said to have replied, "What fool will sell us his secrets?"
Why would the British government agree to such a sale? The USSR soon developed the MiG-15, one of the most successful and widely used military aircraft in history, using these engines. Could they have been that naive to think the Soviets wouldn't convert them for military use or did they have another motive? For example, would having more intimate knowledge of the engines of their jet aircraft have been worth helping in their development?
Sir Stanley Hooker, who with Frank Whittle was greatly responsible for the British jet engine program, said in his memoirs that the decision was political, taken by the Board of Trade under Sir Stafford Cripps. Previously, the British had kept their jets from operating beyond the Netherlands, to avoid the Russians learning about them. The post-war labor government was more friendly. Cripps was left-leaning, Hooker was not, and in a memoir that details many of his dealings with administrators and bureaucrats, it's remarkable that Hooker says nothing of Cripps, who was Minister of Aircraft Production at the end of the war. Clearly, he felt Cripps was wrong.
Cripps had been on a mission to Moscow in the early years of the war and , despite his socialist inclinations, was frustrated by his dealings with Stalin and the Russians. That he would shift to selling them planes with the Neme engine is a little surprising. But he had always been very confident of his own views ( Churchill once remarked "there but for God, goes God") and correctly saw the need after the war to boost foreign sales, in order to pay Britain's enormous war debt: and that sale to the USSR would have been a part. Nor was the jet engine technology jealously guarded : Hooker was also sent by Rolls Royce to Argentina, around the same time, to assist them in developing own jet ( which they didn't). Cripps had enough of a technical background that he may have thought the technology would eventually pass, anyway. And when the Korean War broke out, he was a wreck. He had led an amazingly busy life, and by 1950 he had managed to work himself almost to death, and would indeed die some months later in a clinic. Like everyone else, he would also not have been able to predict the outbreak of the war,, or see the effect of selling the Neme. But this is all rather hypothetical: I have not done a deep dig on Cripps , and while a shallow dig didn't yield any light on this, there may be some scholar who's really excavated..
There's also a problem with technology theft: the thief doesn't have the depth of knowledge gained from the development. Hooker , in the 1970's, was shown a MIG engine by the Chinese, and remarked to them " Yes, they even copied the mistakes".
Stanley Hooker: Not Much of an Engineer