Q: is there any likelihood that increasing the rate to 50% would raise significant amounts of additional tax?
A: "The evidence suggests that raising the top rate of income tax from 45% to 50% would likely raise only a small amount of additional tax revenue, and possibly less than expected.
When the 50% rate was previously in place (2010–2013), tax receipts were lower than forecast because high earners responded by reducing their taxable income—through tax planning, shifting income, or other means.
The Office for Budget Responsibility (OBR) and HMRC found that, after accounting for these behavioral responses, the 50% rate raised only about £0.6 billion per year more than the 45% rate, far less than the theoretical maximum if people did not change their behavior.
The IFS and other researchers conclude that the revenue-maximizing rate is close to 48%, so moving from 45% to 50% would raise some extra revenue, but the amount would be modest and highly uncertain.
If people did not change their behavior at all, the increase could be as much as £3.3 billion per year, but in reality, the actual increase is likely to be much smaller due to income shifting and other responses.
In summary, there is some likelihood that increasing the top rate to 50% would raise additional tax, but the amount would be limited and far lower than a simple calculation suggests. Most official and independent estimates put the extra revenue at well under £1 billion per year, with significant uncertainty."