Pension pot: £1

25% tax-free lump sum: £0.25

Higher income tax of 40% on remaining £0.75: £0.30

Pension pot after income tax: £0.25 + £0.75 - £0.30 = £0.70

Effective tax rate: 30%

This would be reduced a bit if taking into account personal allowance. For example if the pension pot was £100k and the personal allowance £12,500 the effective tax rate would be 25%.