A W8 form is only required for Non US residents who hold US stock (ADSs). If you exercise an option and SELL ALL a W8 is NOT required. You can obtain a W8 here.
A W8 is required by Citibank (Computershare) if you retain stock which is then transferred to be registered on the Direct Registration System (DRS). If a W8 form is not received by Citibank prior to the sale of shares, a dividend payment or dividend reinvestment, Citibank are required to withhold an additional 28% on top of your standard rate to be compliant under the Foreign Account Tax Compliance Act (FATCA).
If you hold shares on the DRS or exercise options and retain ADSs on the DRS, Computershare will contact you to obtain a copy of your personal W8 form.
If you have shares registered in your name on the DRS, please ensure your account is 'CERTIFIED' or you will be subject to the additional 28% tax. In order to ensure that your account is certified, please log on to your account on the Computershare Investor Centre Website.