Well if UK and France decide hold off the war just let Nazi Germany invade Poland the war end at lot sooner since Germany then declare war on USSR. Russia winter would destroy Germany army. France and UK much better shape stop Germany must force dying fighting Russia.
If France and the UK left Poland alone they can basically fire their diplomatic corps as no nations on earth will sign any defensive treaty with them as between Munich and this they had demonstrated to be highly unreliable and i just use some eufemism.
Regarding the attack on Russia, well a lot depends on when Germany attacks, an immediate invasion after the conclusion of the Polish campaign is doubtfoul as the German army need time to rest and resupply and uncle Adolf may not feel to be ready. No war with Uk and France will butterfly away the Greece and Yugoslavia campaign or at least it will be only an affair between Italy (plus Hungary and Bulgaria) and Yugoslavia so Germany will not feel obliged to intervene and any invasion plan will not be derailed...on the other side Uncle Joe wil probably a lot more suspicious of Hitler and not dismiss outright any rumors of German attack.