"Astonishingly, the grave errors committed by the British High Command at the Battle of Loos were not learnt from and were to be repeated on the first day of the Battle of the Somme which ended on 1 July 1916 in the greatest disaster in the history of the British Army."

Hmmm grave errors committed by the British High Command? Or grave errors imposed upon the British High Command at the insistence of the French Generals who when all said and done were at all times formed the Supreme Allied Commanders in France.

At Loos in August 1915 Haig told Sir John French that the area assigned for his First Army's attack by Joffre was far from suitable (As described by the French General), Haig also advised Sir John French that for the plan to have any chance at all for success the Reserves had to available instantly so therefore had to be held close to the front. Sir John French ignored Haig's advice - As you will no doubt refuse to take my word for it then read the words of a man who was there:

"A great deal of nonsense has been written about Loos. The real tragedy of that battle was its nearness to complete success. Most of us who reached the crest of Hill 70, and survived, were firmly convinced that we had broken through on that Sunday, 25th September 1915. There seemed to be nothing ahead of us, but an unoccupied and incomplete trench system. The only two things that prevented our advancing into the suburbs of Lens were, firstly, the exhaustion of the "Jocks" themselves (for they had undergone a bellyfull of marching and fighting that day) and, secondly, the flanking fire of numerous German machine-guns, which swept that bare hill from some factory buildings in Cite St. Auguste to the south of us. All that we needed was more artillery ammunition to blast those clearly-located machine-guns, plus some fresh infantry to take over from the weary and depleted "Jocks." But, alas, neither ammunition nor reinforcements were immediately available, and the great opportunity passed.
— Richard Hilton, who was present at the battle acting as a Forward Artillery Observation Officer.

In March 1915 the British First Army under Haig attacked at Neuve-Chapelle, All objectives were taken but Sir John French kept the reserves too far to the rear so that they could not deployed when they were needed to exploit the break through - The Germans mounted numerous counter attacks and lost heavily - Jom listen to this next bit, something that none of your 1929 to 1969 historians and playwrights ever heard about because it wasn't known till much later once German records were researched and translated - Neuve-Chapelle so frightened the Germans that it became policy after this battle that for the German Army when facing British Troops the lines of defence had to be doubled in strength and prepared in depth. That was the factor that met Haig's First Army at Loos and once again they very nearly succeeded. Two fuck-ups to Sir John French so he was sacked and replaced by Haig (Under the Buggins's turn system prevalent in the pre-war British Army the job should have gone to Robertson, but he had no experience of combat in France against the Germans, Haig had by now given the Germas a fright on two occasions so at Robertson's insistence Haig took command of the Army in France and he accepted the job of Chief of the Imperial General Staff)

The Somme in 1916 again was a fight picked for the British Army in France by British Politicians at home [David Lloyd George] and French Supreme Commanders with the primary objective of relieving pressure on the French defending Verdun. As at Loos Haig argued that to attack on the Somme was to attack at the wrong place place - Haig wanted to attack in Flanders in 1916, but again as at Loos Haig was over-ruled. The attack had to be made in conjunction with the French and that meant the Somme. However things were going so badly for the French at Verdun that roughly half the French troops who were supposed to have been attacking with the British on the Somme on the right flank of the British were withdrawn and sent to Verdun instead, as the "junior partner" Haig had no say or leverage in the overall scheme of things. The German Commander in the west in 1916 Falkenhayn started the year off promising to bleed the allies white using simple attrition - by the end of 1916 it was the Germans who had been bled white and Falkenhayn was dismissed - the Germans after 1916 knew that they could not defeat either the French or the British on the western front until after they had defeated the Russians in the east. The 1st of July 1916 might have been a bloody day and a disaster for the British Army, but 1916 ended up as being a bloody year for the German Army and a year that they never recovered from, we on the other hand did. It also caused the Germans to build and withdraw to the Hindenburg Line

And as you mentioned Passchendale Jom here we have another instance of David Lloyd George's meddling. Lloyd George completely mesmerized by the promises of the planned Nivelle offensive ordered Haig to attack at Passchendale in order that the ports on the Belgian coast being used by German U-boats could be captured, Haig argued that the ground on the Somme would be better for the tanks he know had at his disposal - Once again the advice and opinion of the man tasked with doing the job was ignored by those sitting hundreds of miles away from the action.