According to this article (yes, it is the hated DM);
www.dailymail.co.uk/news/article-2239534/Millionaires-exodus-Two-thirds-Britains-earners-deserted-UK-50p-tax-rate-introduced.html
The 50p tax rate brought in by Labour in 2010 resulted in a loss of £6.9 billion in the year 2010/2011, because there was a drop in the number of millionaires paying the 50p tax rate from 16,000 to just 6,000 (i.e. they either left the country or reduced their taxable income).
Do people think it is better to introduce an ideologically driven tax on earned income for very high earners, even if it means there is actually a net loss in tax take for the government? If so, why?