It's theory, but high earners might not earn so much as a result of Brexit, which means everyone else has to top up the tax they would have paid.
Slowing income growth among the richest 10 per cent of earners could disproportionately affect the UK tax take, according to research on the economic consequences of Brexit. The richest tenth of households pay 30 per cent of the biggest taxes — income tax, national insurance (NICS) and value added tax (VAT), according to the Institute for Fiscal Studies, an independent think-tank. It has warned that the rest of the population would be forced to pay more tax if top earners saw their pay stagnate or they moved out of the UK. The government relies heavily on richer taxpayers for income tax, although this is less true of NICS and far less true of VAT.