As you may be aware, the UK has recently enacted strong age-verification legislation under the UK Government Online Safety Bill. Some potentially useful information can be found here; the second link includes specific information that lists acceptable means to verify your age:
When updating to iOS/iPadOS 26.4 or later, it is necessary to complete a one-time robust age-verification process - the required steps being determined by how much information Apple already possesses about you and your Apple Account. In some circumstances, Apple will verify your age using a Credit Card registered in your name.
Providing Credit Card information, where available, facilitates easy and reliable age verification of the card-holder via a "soft" Credit Check. Old-style "paper" UK Driving Licenses cannot be used for age verification.
If you are experiencing difficulty with the process, you would be well advised to seek assistance from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page.
Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044
If you are unable to complete the verification process, the only immediate consequence will be inability to access to some online 18+ content and some social media.
Apple has also provided the following information:
The UK has specific age assurance requirements to access certain content. You may be required to confirm you’re an adult before changing certain safety settings or taking other actions with your Apple Account. To confirm you’re an adult, a credit card, ID, or other method may be used. If you’re a teen, you might need to join a Family Sharing group so a parent or guardian can help manage your safety settings. If you’re a child, a parent or guardian is required to create a Child Account for you and add you to their Family Sharing group, so they can manage your parental controls and settings.
If you’re asked to confirm you’re an adult
Age requirements for managing an Apple Account in the UK
Country-specific rules for managing an Apple account
Family Sharing overview for kids and teens