The Grapple X on 8 November 1957 (a few months after the Grapple 1 on 15 May 1957) was a successful thermonuclear device[0], so it was only a short term bluff.
"Implementing the McMahon Act created a substantial rift between United States and Britain. The new control of 'restricted data' prevented the United States' allies from receiving any information, despite the fact that the British and Canadian governments, before contributing technology and manpower to the Manhattan Project, had made agreements with the United States about the post-war sharing of nuclear technology. Those agreements had been formalized in the 1943 Quebec Agreement. In the case of the United Kingdom, these were developed further in the 1944 Hyde Park Agreement, which was signed by Winston Churchill and Franklin Roosevelt"
Bear in mind that this was in the context of the British feeling betrayed by the Americans on nuclear research - from https://en.wikipedia.org/wiki/Atomic_Energy_Act_of_1946 :
"Implementing the McMahon Act created a substantial rift between United States and Britain. The new control of 'restricted data' prevented the United States' allies from receiving any information, despite the fact that the British and Canadian governments, before contributing technology and manpower to the Manhattan Project, had made agreements with the United States about the post-war sharing of nuclear technology. Those agreements had been formalized in the 1943 Quebec Agreement. In the case of the United Kingdom, these were developed further in the 1944 Hyde Park Agreement, which was signed by Winston Churchill and Franklin Roosevelt"
[0] https://en.wikipedia.org/wiki/Operation_Grapple#Grapple_X