What are you referring to here? Higher rates of income tax are only taken on the money earned over the band. So If you earn £50,271, you pay 20% on £50,270 and 40% on £1.
Are you referring to some other kind of tax then?
The marginal tax rate for 100k -> 125k is 60% (due to losing the ~£12k tax free allowance)
"Your personal allowance goes down by £1 for every £2 that your adjusted net income is above £100,000. This means your allowance is zero if your income is £125,140 or above."
Perfect.