If you're UK based and not particularly interested in receiving emergency alerts tomorrow, there's a straightforward way to turn them off:

Android

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Catch-all

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Since these settings can vary between OEM and Android version:

  1. Head to Settings
  2. In the search bar, type Emergency alerts
  3. Jump in to the relevant page
  4. Toggle off Allow alerts for everything, or the individual alert types as desired

Modern Android

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  1. Head to Settings
  2. Tap into Safety & emergency
  3. Tap into Emergency alerts
  4. Toggle off Allow alerts for everything, or the individual alert types as desired

Huawei

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  1. Head to Settings
  2. Tap Sounds & vibration
  3. Tap More settings
  4. Tap Cell broadcasts
  5. Toggle off Emergency Alerts or Extreme threats and/or Severe threats depending on your EMUI version

Xiaomi

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  1. Head to Settings
  2. Tap Passwords & security
  3. Tap Emergency alerts
  4. Toggle off Extreme threats and/or Severe threats as desired

iOS

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  1. Open Settings
  2. Tap Notifications
  3. Toggle off Emergency Alerts and/or Severe Alerts (near/at the bottom)

Windows Phone

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  1. Open Settings
  2. Tap System
  3. Tap Messaging
  4. Tap Change emergency alert settings
  5. Toggle off Extreme threats and/or Severe threats as desired