Scrutiny via a representative body. It uses its powers to ask questions of the government to gain answers to any concerns that it may have, on its own account or on behalf of constituents.
Scrutiny via the mechanism of judicial review. The judiciary examines the legality of government action and may require action or prohibit actions that it holds to be illegal, irrational, procedurally improper of a breach of the HRA 1998.
Scrutiny by the public, often via means of media reports. The public get to chose their MPs, and indirectly the government through parliamentary elections.
Power has been conferred via Parliament through Acts of Parliament and delegated legislation.
Power has been conferred through the common law and is vested in the Crown.