Jim, Britain did not enter the war for imperial gain.
It was intending to keep out of it.
The invasion of Belgium was not expected, and so neither was our entry.

As a consequence the army was unprepared and ill equipped to face the German onslaught.
We fought to resist the invasions which also threatened our own security.

Of course there were encouragements to join, but the numbers volunteering overwhelmed the capacity of the army to process them anyway.

The numbers volunteering increased dramatically when the British Army was thought to be defeated and running, and total defeat imminent.
Any prospect of winning seemed years away and imminent defeat far more likely.

When there were just not enough left to volunteer, conscription was brought in but it did not produce many extra men. Those who had not volunteered mostly had valid reasons not to go. It was necessary to lower the physical requirements and to admit older men.