If Haig had worked out that strategy in say 1916 and converted the British Army into a flexible, mobile, adaptable well-trained fighting force,
Not asking a lot are you?
1) By "that strategy" I take it you mean basically Blitzkreig? It took the German army 4 years to work that one out and they were a land based military.
2) The British Army was tiny, the original BeF was 100,000 men. Very good but very small. Covert what? The Army didn't start recruiting until 1914. They had to go into the front to relieve the pressure on the French as soon as possible.
3) The equipment didn't exist in 1916 to fight like they did in 1918. The artillery wasn't accurate and reliable enough for one thing.
By 1918 the British were using very advanced creeping barrages with accurate guns, and combined air power working with the infantry. It beat the Germans then and it was basically what the allies did in 44 45.
The war was brutal and savage. With hindsight anyone could do it better but it needs to be considered in the context of its time.