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.
William Eagleson Gordon VC - the battalion was ordered (but didn't receive the orders) to retire. Gordon came to the view that the position was hopeless (as had the Brigade commander) and believed that the battalion should seek to break out to regain British lines. The CO demurred, whereupon Gordon pulled rank and an attempt to break out was made - but it was too late and most of the survivors were captured.
Gordon's health declined in captivity and he was exchanged for a Prussian prince in 1916. After the war, one of the men was quoted as saying that Gordon had ordered the battalion to surrender (this in an article in The People's Journal). Gordon sued for libel and won; it was alleged that the CO and members of his family held a significant enough shareholding in the newspaper to put the editor up to blaming Gordon for the surrender, but as well as winning the court case he was exonerated by a Court of Inquiry into the events of 1914 (which published its conclusions shortly after the court case ended, agreeing that Gordon hadn't surrendered and had, indeed, wished to attempt a break out.
In terms of Brevet, LJ is absolutely correct about the etymological origins - but the term extended to mean the badge thanks to the original practice in the RFC of presenting wings and flying certificate to a newly-qualified pilot. The new pilot had thus been awarded his 'flying brevet'... and over time, this came to be interpreted as the badge which was part of the original 'brevet' package, as it were.
One hardly dares point out that the RAF Club website now states that affiliate membership is open to "A flying brevet wearing officer of the Navy or Army'...