I've no idea whether brevet rank is still conferred in the British Army but, in 1914, it appears to have been an Army, rather than regimental, concept. 1st Gordon Highlanders deployed to France in August 1914 and, in the retreat from Mons, took part in the holding battle at Le Cateau on 26 August. They did not receive the withdrawal order that afternoon, and neither did companies of other regiments on each side of them. As the situation developed, a Gordons Lt Col (Brevet Colonel) - not the Gordons' CO - claimed that as those left constituted an Army grouping, it fell to him to take command. I imagine that this caused some disagreement with the CO but that's what finally happened and the group began to withdraw in very difficult circumstances after midnight - my maternal grandfather died in the ensuing confusion as they encountered German forces that were now surrounding them. Some years after the war, I believe this incident led to a legal case in the Scottish courts.
Whatever merits this system may have had in a peacetime army, my impression is that it did not transfer well into the heat of battle.