The thing I got from watching They Shall Not Grow Old, was how those men actually looked forward to fighting. Life for the working class at that time was so monotonous and dreary that they welcomed any diversion from it. War was the “great adventure”.
When they mention most grew an inch in height and gained a stone in weight I thought some was due to underage recruits (19-35 years old,legal age) but I doubt all of them were getting 3 meals a day before than.