If you are a member of Her Majesty's armed forces, a Crown Servant or a British council employee you can apply to be included on the register by declaration or on a household registration form as a service voter. Renewal forms are sent out three months prior to the renewal date
If a service voter registers as a 'normal' elector, they effectively cease to be categorised as a service voter. The declaration must be submitted via the relevant government department or the British Council.
Members of the armed forces and their spouses may register: 

•  By service declaration (for a period of 5 years). The declaration must be received by the Electoral Registration Officer within  three months of the date of the declaration and may be cancelled at any time by the declarant; or
• At their home address (in the same way as civilian electors - using individual application forms or the annual canvass form)

Please contact the electoral registration service to request a form.