Britain’s next Prime Minister will have the dual challenge of leaving the European Union and setting up the newly independent country for economic success. So it’s a good sign that some top contenders for the Conservative Party leadership—the winner will replace Theresa May—started with tax reform as campaigning began on Monday.

Britain’s Byzantine tax system still drags on investment, productivity and growth despite important recent improvements. The top corporate rate has fallen to 19% from 30% since 2007 and is due to hit 17% next year. But the top personal rate, paid on incomes above £150,000, has fallen only to 45% from 50%.

Coupled with abrupt income cutoffs in eligibility for allowances and credits, British taxpayers in practice can experience a marginal rate as high as 60% for each additional pound of income between £100,000 and £124,000, and 65% for families with three children earning between £50,000 and £60,000, according to the Institute for Fiscal Studies.

Add taxes on pension contributions at higher incomes and some workers pay marginal rates above 100% on parts of their income—paying more than a pound in tax for each additional pound they earn. There’s evidence that some workers, especially experienced doctors, are reducing hours to avoid this trap. Social-insurance and property taxes add more burdens.

The best Conservative leadership proposals so far come from Foreign Secretary Jeremy Hunt and Home Secretary Sajid Javid. Mr. Hunt pledges to reduce the corporate rate to 12.5% to match Ireland’s low rate, though he wouldn’t touch personal rates where the worst distortions lurk. Mr. Javid would cut the top individual rate to 40%.