[Sholmesy]

As well as the childcare benefit removal others have mentioned, you also begin losing your "tax free allowance".

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."