Three of the Big Four are head-quartered in London†, and so the most likely avenue to better regulate them would begin in Westminster. But Westminster has for many years been in the hands of Tories, who have successfully hoodwinked the British people into believing that somehow all the awful things Tories keep doing would be fixed if only they had more Tories in charge of everything. The Tories pay the Big Four piles of tax payer money, including to "fix" problems caused by other members of the Big Four. For some reason the proposed answer often seems to be "Pay Big Four firms more money" perhaps they could pay a Big Four firm to find out why that is?
† All of the Big Four are organised as Groups, so that a relatively small company - in three cases based in London - handles the global brand, while independent businesses in dozens of countries share that brand, intellectual property and so on. Legally it is possible to pretend these aren't the same firm, although it sure is convenient how easily senior employees move between one of the independent businesses and the others as they wish...