British Commanders continuously fucked it up
it was'nt so much fucked it up as the technology had advanced so quickly that the tactics (used by both sides i might add) became so outdated and hideously costly
when the tech was single action rifle fire with slow firing cannon artillery then the tactics of cross as fast as possible while staying together to bring maximum pressure to the contact point was a workable one
machine guns, barbed wire defences and quick firing artillery meant that it became a game of 'can our artillery keep their heads down so that we can get across NML quickly enough before they can respond to the attack' - hence rolling artillery bombardments.
did it take too long for the generals to realise that they were dinosaurs in a machine gun age - hell yes
were British combined ops the best in the world by 1918 - again yes, they took the best of the german stormtrooper concept, added tanks to the mix and were the top dogs at the end of the war
(then proceeded to forget all this in the interwar years - but thats for a different discussion)