In Hillingdon, there isn’t a specific deadline for registering to vote anonymously or opting out of the open register; you can do this at any time.
You can opt-out during your registration process or at any moment afterward by reaching out to the electoral services office. It's good practice to make this change as soon as possible if you're concerned about your privacy.
For more information on how to proceed, you can visit the Hillingdon electoral services page.