The British army went to war in 1914, not only a small, basically Colonial enforcement organisation, with limited numbers, but with ideas of warfare, as most powers in the world did, of fighting a war along lines of the older conflicts. It could be said to be damming that all the major powers seemed to not understand fully that the concept of warfare had moved on.
The Russo/Japanese war had shown that trenches and machine guns were more dominant than they had been previously, yet whilst the machine gun was introduced it was done so in limited numbers. The British Army was perhaps less enthusiastic about this than most, going to war with only two machine guns per battalion.
The way that the British Commanders learnt how to adapt to a war they were not prepared for, principally an army of millions rather than hundreds of thousands, fought with non regular soldiers across a stagnant landscape littered with barbed wire and protected by machine guns, is usually called the 'learning curve'
The learning curve is perhaps best explained by the term 'ambling walk up and down dale' rather than curve. the way the Generals learnt was not always in a rising line. What was learnt often had to be re-learnt or slightly adapted. What worked one time did not another for the parameters had changed. As new concepts came in, Tanks for instance, then tactics could or had to change again to adapt to this. Then a lesson learnt earlier in the war could be used etc.
Not all the Generals were good at this learning curve, many were vastly out grown by this transformation in warfare, yet many also conducted themselves with great prowess and astuteness.
A factor not widely accepted as a factor in this learning curve was the lack of qualified Staff officers, those trained at either Camberley staff college or the Indian equivalent, Quetta. 1914 had taken a great toll on the experienced officer corps and it would be years before the volunteer officers could make up the deficit in this area. As such staff work was often learnt on the job, with the obvious results of mistakes, errors and apparent bungling.
Regardless by 1918 the British Army had grown into a massive, highly competent fighting machine and played the major part in the destruction of the German army during what is known as the 100 days. It had learnt via the Somme, 3rd Ypres, Cambrai, German spring offensives and many other smaller battles, the hard way. But by the end they had moved from a cavalry/infantry dominated army to an organisation that could put a two million strong army in the field, adapt it to machine guns, tanks, gas, aeroplane combined operations and many other tactical concepts to win a conflict the likes of which had never been seen previously, nor perhaps in innovation will be again. For the Great War was the only conflict, for Britian, to be fought with a million plus strong army, where the commander did not have access to the whole battlefield around him, able to issue direct real time orders, or alternatively a suitable radio/telephone communication, as in WW2. Perhaps that they adapted to this over came and won is in itself a miracle