How Soviet alignment could deter an invasion of Poland

Would Britain still oppose a German invasion of Poland while fighting Japan?

Yes, and Soviet alignment with the Allies could make the warning more effective by denying Germany the eastern security it historically obtained before invading Poland.

The decisive change is the Soviet Union's place in the anti-Japanese coalition rather than simple British distraction. Germany attacked Poland historically only after securing Soviet cooperation and avoiding an immediate eastern front. If Moscow were already aligned with Britain and France in the Pacific war, Berlin could face France in the west and the Soviet Union in the east. The Allies could therefore issue a more credible warning against opening a European conflict while they fought in Asia. Hitler would be unlikely to choose Germany's First World War two-front predicament under those conditions.