Iirc Dan Carlin said in his podcast that the English army consisted of 90 000 men when they landed in Europe. How could they have made a difference with such small numbers compared to the other powers? Looking at losses other powers suffered, why didn’t the English army simply melt away?
A good answer on this topic can be found here.
Also, u/TheBobJamesBob wrote about the size of the British Army during the First World War here, and there is another answer on the growth of the British Expeditionary Force here (They don't directly answer your question but hopefully you will find them helpful).