That's the question millions are asking, or at least the one they should be asking. The OECD recently did an analysis of the economic consequences for the U.K. if it decides to leave the European Union. It concluded that it would cost the country 5.1 percent of GDP in its central estimate. Other analyses have arrived at similar estimates. Such estimates have been cited by right-thinking people everywhere as a powerful argument against the U.K. leaving the European Union. (It is.)
But Brexit is not the only policy that can cost the U.K. large amounts of output. Since the Cameron government came to power in 2010 it has placed a priority on reducing the budget deficit rather than restoring the economy to full employment. As a result, the U.K. economy is still well below its potential level of output. (The potential has undoubtedly fallen as a result due to weak public and private investment since the downturn and workers experiencing prolonged stretches of unemployment and thereby losing skills.)
To get a simple estimate of the output lost due to Cameron's austerity policies, we can compare the I.M.F.'s projected growth path for the U.K. from 2008, before the severity of the recession was recognized and its current level of output. In 2008, the I.M.F. projected that the U.K. economy would be 26.2 percent larger in 2016 than it was in 2007. (Since the projection only runs to 2013 I have assumed that the growth rate for 2012 to 2013 [2.7 percent] continues for the next three years.) The most recent projection shows that 2016 GDP will be just 9.4 percent higher than the 2007 level.
If we can credit the I.M.F. research staff for knowing what they were doing in their 2008 projections, then the U.K.'s austerity policies have cost it an amount of output equal to 16.8 percentage points of 2007 GDP or more than three times the estimated cost of Brexit. This means that if Brexit is an economic disaster then Cameron's austerity has been three times as costly as an economic disaster.
For the curious ones out there, the I.M.F's projections showed the U.S. economy being 26.4 percent larger in 2016 than in 2007. The most recent projection shows the economy being 12.6 larger. The implied loss of 13.8 percentage points of 2007 is a bit less than the three times Brexit measure.