If the options are exercised at at a discount to the market value at the time of exercise then you are liable for income tax at your marginal rate (0%, 20% or 42%) on the difference.
Once you have exercised the options then when you sell them you are liable for [broken link removed] (20%) in any gain that you make (less allowance, less allowed expenses etc.).
If you exercise and sell on the same day then you are only liable for income tax on any discount to the market value and there are no CGT implications.
You must file and pay these taxes yourself - i.e. they are not taken care of automatically via payroll or whatever.
Does that help?