If it is like my scheme, you are taxed at your marginal rate at the time that you leave.
So it is as if the value of the shares have been added to your salary/other benefits this year. If that would push you into a higher tax band then you will pay more tax.
If you are a higher rate tax payer and won't be pushed into additional rate or the personal allowance clawback, then you will be taxed 42% for tax and NI.