The reason the British Army retreated so quickly is partly because of Shire's warning.
Shire accurately predicted the movements of the German Army, but the British Commander-in-Chief Haig ignored it. Therefore, when the German attack became a reality, any remaining trust and hope the British officers and soldiers had in their commander vanished completely.
If you don't trust your commander, nor his abilities, naturally, there will be no morale for fighting.
This left the British Expeditionary Force almost one-sided in the ensuing defense battle, being driven back like sheep by the German Army.
The other reason is that the Germans timely unveiled the K-Wagen super heavy tank.
This tank actually just looks intimidating; it has no real advantage in tank warfare, with a top speed of only 7.5 kilometers per hour.
This speed is only slightly faster than walking.
