These are the initial postulates:
A) Barbarossa starts in Europe on June 22, 1941
B) Japan is launching an attack on the USSR at any point from late July 1941 to early October 1941,
in the meantime, up until B) happens,
C) The US has not imposed an oil embargo or asset freeze on Japan
D) Japan has not advanced in Southeast Asia since occupying Tonkin in 1940
the primary question is how does Britain react to the attack on the USSR by Japan?
Britain and the USSR are in a partnership in Europe against the Nazis. Their military cooperation extends at least as far east as Iran. Soviet survival and continuation in the war is a compelling British interest.
With this in mind, does Britain start hostilities against Japan, initiating economic war and attacks on Japanese shipping.
This exposes British interests in China and Hong Kong to instant retaliation and other British possessions in Southeast Asia and the Pacific to certain retaliation. In the meantime, Britain would incur this retaliation while not able to cause significant military damage to Japan, especially in the first few months of opening hostilities.
All that considered, what are the British likely to do once the Japanese attack the Soviets?
Secondarily, how will the Americans react?