Your salary is in the Personal Allowance taper zone. For every £2 you earn above £100,000, you lose £1 of Personal Allowance. This creates an effective 60% marginal rate in this band.
Your estimated Income Tax on £125,000 is £37,488, giving you an effective Income Tax rate of 30.0%. Combined with National Insurance of £4,511, your total deductions are £41,999, or 33.6% of your gross salary.
Since your salary exceeds £50,270, the portion above this amount is taxed at the Higher Rate of 40%. National Insurance also drops from 8% to 2% above this threshold, providing a small offset.
Personal Allowance Taper: Your income exceeds £100,000, so your Personal Allowance is reduced from £12,570 to £70. This creates an effective marginal rate of 60% in this band, as you lose £1 of tax-free allowance for every £2 earned above £100,000.