The situation is probably somewhere between the extremes. As said, the UK was not going to allow either France or Germany a free hand to gain hegemony over the continent. There was still a lot of bad feelings about Germany over WWI, but the population had to be well aware of the various earlier issues between the UK and France, so it's not a 100% versus 0% popularity situation. France would get a bit more slack than Germany, but not a free hand.
In my opinion, a incursion into Germany by France, with a Franco-Soviet alliance in place, would be threatening enough to drive the UK to take diplomatic and possibly economic action, but short of military intervention of any kind, against France. If things go badly for Germany, the UK might even provide non-military aid. If France violated the neutrality of Belgium or Luxemburg in the process, or the Soviets decided to attack Germany through Poland without permission, then I see matters very likely escalating. Otherwise, I don't see any realistic chance of the UK joining Germany against France if the French restrict their incursion and subsequent demands to the Rhineland area.
In my opinion, a incursion into Germany by France, with a Franco-Soviet alliance in place, would be threatening enough to drive the UK to take diplomatic and possibly economic action, but short of military intervention of any kind, against France. If things go badly for Germany, the UK might even provide non-military aid. If France violated the neutrality of Belgium or Luxemburg in the process, or the Soviets decided to attack Germany through Poland without permission, then I see matters very likely escalating. Otherwise, I don't see any realistic chance of the UK joining Germany against France if the French restrict their incursion and subsequent demands to the Rhineland area.