The use of hollow-point or 'dum-dum' rounds in warfare was prohibited by the Hague Convention of 1899 and this prohibition remains in force. It's quite legal, however, for States to use them against its own citizens for purposes of 'law enforcement', and this is just what the Met has done. In future all 9mm rounds used by Met firearms plods will be dum-dum.
The use of hollow-point rounds in putting down colonial insurrections was defended by Sir John Ardagh in the following terms "The civilized soldier when shot recognizes that he is wounded and knows that the sooner he is attended to the sooner he will recover. He lies down on his stretcher and is taken off the field to his ambulance, where he is dressed or bandaged.. Your fanatical barbarian, similarly wounded, continues to rush on, spear or sword in hand; and before you have the time to represent to him that his conduct is in flagrant violation of the understanding relative to the proper course for the wounded man to follow - he may have cut off your head". In warfare against savages, Ardagh said, a bullet that caused such massive tissue damage and loss that it would physically stop the fanatical tribeman was needed.
The only problem is that the Met can't seem to tell the difference between a fanatical jihadist and a suntanned tourist, or indeed between a fanatical jihadist and a fat Irishman carrying a chair leg. And the keen volunteers from whom the Met's firearms plods are selected are by definition the least suitable persons to be allowed to carry firearms; you don't become a farmer because you like killing rabbits.
Still, the 'stopping power' of a score of plods armed with H&Ks loaded with dum-dum against a violent anti-government protest must give the political class a little more comfort today.