Kevin Drum links evidence that the British media seem to be much more hardball in its questionning and insistent that politicians answer the question asked than their American counterparts. There are some good responses in the comments section so far as well.
I would argue the differences in the media expectations of candidates is largely explainable by the institutional nature of the two political systems. The US system is one of divided government, both in terms of separations of powers among three branches at the national level but also federalism and its geographical dispersion of power. The British PM and his
ministers are therefore much more accountable in their system than any given politician in ours.