Britain definitely isn't going to war over the Soviet invasion of Poland. The guarantee had been specifically phrased that way. The best opportunity is Finland: after that, Britain does seem to have been making very serious plans, but there's no reason for Nazi Germany not to gum them up by launching the blitzkrieg.
The Soviets were widely regarded as a malevolent neutral or pseudo-Axis in the earliest phases of the war (Churchill was being typically contrarian when he made his "mystery inside a riddle" speech asserting that Russian and German interests fundamentally clashed, that Russia was acting on its own behalf, and that it wasn't necessarily Britain's enemy). What kept the peace was that Britain and France had their hands a bit full, and Stalin wasn't stupid enough to provoke them (note that he waited until Germany was on the attack before he gobbled up the Baltics and Besserabia; peace in Finland came when it did in large part because Stalin didn't want to risk a war with the Entente). But as pointed out, bombing Baku was certainly mulled.
Now, if everyone screws up and there is a war over Finland, Stalin obviously wants to end it as promptly as possible. What does he have to gain? The only place he can really strike an Entente interest is Iran (grandious, logistically fantastical schemes to invade India all came from Germany during the doomed Axis negotiations). I'd imagine the Soviet-Entente war would itself be "phony", except of course in Finland.
I'm not sure how Finland would unfold (you might want to ask a Finn), but once Germany's beaten France, I don't see how the Entente can sustain themselves there. After that, the diplomacy gets interesting, as Stalin was already extremely wary of Germany and confident of her eventual betrayal.