Interesting discussion about it on Quora:
https://www.quora.com/In-1946-the-UK-government-agreed-to-sell-Rolls-Royce-Nene-jet-engines-to-the-USSR-as-long-as-they-dont-use-it-for-military-purposes-The-Soviets-quickly-started-putting-them-into-jet-fighter-aircraft-Was-the-British#:~:text='In 1946%2C the UK government,British government really that naive.”
I tend to agree with the first guy:
The Cold War did not really kick-off until 1947, and the Labour government in the UK were not knee-jerk anti-communists like most American politicians.
Moreover relations between the US and UK were not warm. The Truman administration had abruptly terminated Lend Lease after Japan had surrendered, forcing the UK to negotiate an emergency loan from US banks. This was considered to be an economic attack on the Labour government and its policies.
It was therefore considered to essential to maintain an amiable relationship with the Soviet Union. If only to stop the US from taking us for granted.
Selling the Nene to the Soviets was regarded as a good will gesture without great strategic significance: the Soviets already had access to German jet technology and the British had decided to concentrate future jet development on axial flow engines rather then centrifugal compressor engines like the Nene. If the Soviets did renege on the no military use clause and reverse engineer the Nene it would give them a temporary benefit but it would also send them down a technological dead end.
Still, it did allow the Soviets to field the MiG-15 sooner than they would have been able to without having the Nene to copy and it turned out to be a nasty surprise for the US.
BTW: This book is a pretty good read and he makes a strong case that the supposed 10 : 1 kill ratio of the Saber over the MiG was basically BS: