According to the income levy FAQ ([broken link removed]) you are not obliged:
1.14 Am I obliged to include the income levy due on share option gains when making my
payment of RTSO (Relevant Tax on Share Options)?
No, there is no obligation to include the income levy when making a payment of RTSO (see Information
Leaflet CG 16 for more information on RTSO).
Payment of income levy should be made at the same time as preliminary tax with any balance paid when
the return for the relevant year is filed.