The idea is that the British issue no guarantee – that is, after the disappearance of what remained of Czechoslovakia. As to their needing another policy, that may be true, or not.
If it's true, the alternate policy might be to remain with their hands free. Without guarantees, they can still find another reason to go to war when that suits them – and given Hitler's attitude, he'll be giving them plenty of other reasons down the line. This choice, however, carries a strong risk that the French will feel left alone to deal with the German threat to Poland, and it's difficult to case what they'll do if not supported by the British.
It is also possible that the British do not feel they need another policy for Germany. The point of the British policy on the continent has always been to prevent any one power from gaining hegemony or supremacy. Now, most people today agree that Nazi Germany was indeed hell-bent on that. But did Halifax, for one, share that assessment? What if the British Cabinet concluded that it wasn't so? What if they thought that there was indeed one continental country more dangerous than Germany, i.e., the Soviet Union?
Seraphim74, I agree with your assessment as to the Polish stance. Had the government accepted Germany's demands, they would have faced very strong internal opposition. There were plenty of extreme-right nationalism in Poland at the time. There would certainly have been demonstrations in Warsaw, maybe cases of insubordination in the army, and probably ethnic-related rioting in places like Bydgoszcz. The latter would have offered Hitler an excuse anyway.
Besides, given the Czech case, everybody knew what happened if one acquiesced to Hitler's initial demands; more demands would follow.
Tony Williams, I'm surprised with the British historian. He doesn't sound like one who really studied the German Blitzkrieg in France and the Netherlands. Just deploying that BEF _exactly_ in the path of the German so-called juggernaut, i.e., at the exit of the Ardennes, would more than do the trick. Especially if we add a Belgian historian to tell his Belgian guys to fortify the upper end of Eben-Emael, but that would be icing on the cake.