The Merry Prankster
Pactum serva
I appreciate its not a total myth, but it is vastly over-played. There is no hard evidence to suggest the lower than expected tax receipts were due to people simply upper sticks and moving abroad. Can you give me any figures for the reasons why tax receipts were lower? If not it is just theory. Given the 50p rate came in 2010 and the economy has been steadily declining since then I imagine this is a major factor.
There is also the idea that when people believe their tax rate to be "fair" they pay it when it becomes "unfair" they take steps to avoid it. As 50% is a pretty symbolic amount it maybe that it is a tipping point for such behaviour.