because the UK has the printing presses rolling. There is a deal between the govt and the treasury, austerity in return for letting the printing presses go wild. The UK is basically doing the same as the USA, it may work in the short term (low interest rates) but it will blow up some day. The USA can not keep running near double digits deficitsWell since we're on that topic, explain the sharp drop in 10 year US bond yields from when Cameron took office. If you watch US and UK bond trends since 2008 you will notice that they follow a very similar pattern, which should probably suggest a mild effect on bond rates by Cameron's government at best.
US Debt/GDP ratio has been significantly higher than the UK's during this entire period. From about May 2010 to present, UK bonds went from around 3.75% to about 1.75%. US bonds went from about 3.5% to 1.75%. As prospects of a global economic recovery have dimmed again, bond rates have fallen again. This is most likely because we're in a liquidity trap. During the same time, US growth has continued at a modest (but by no means great) pace, and the UK has returned to recession. Is that the austerity effect you're looking for?
Germany has engaged in basically no austerity measures. Its economy has done quite well, partially due to the same reasons why southern Europe is doing so poorly. The Euro is allowing it to export at an artificially low currency rate.