Britain became skilled in counterinsurgency post ww2 during the decolonisation of Asia. Historically, for a time they were second to none in field battles eg Waterloo. Constant drilling meant they were well disciplined in battle; keeping formation, holding the line, reloading and firing weapons while under fire etc. It was this that led to problems in the Boer war, and also the American war of independence. When the enemy used guerrilla tactics instead of engaging in the pitched battles they were trained for, they were at a bit of a loss how to respond.