Here is a good example of an interrupted time series - the graph says it all. The question is whether pay for performance improved the control of hypertension in British general practices. The answer is no, but this may be because the practices were already doing such a good job that there was little room for extra improvement.
According to this editorial, the UK does particularly well in control of smoking. cholesterol and hypertension compared to other European countries, for both asymptomatic patients (primary prevention) and those with known coronary disease (secondary prevention).