LONDON — Senior executives of the Swiss banking giant UBS faced tough questioning from British politicians Wednesday over a recent rate-rigging scandal that led the company to pay $1.5 billion in fines to global authorities.

During almost three hours of testimony, Andrea Orcel, chief executive of UBS’s investment bank, and the firm’s chief risk and compliance officers were questioned over why the illegal activity, conducted over six years through 2010, had not been discovered earlier.