Presumably your device was purchased within the UK and/or your Apple Account is set to the UK region?
Regional restrictions are necessarily dictated not by your individual nationality, but also reportedly by the region-specific device model number of the device. Having a different nationality, or in using the device in other geographic locations, will not change or alter any region-specific restrictions or applied policies applied at device level. Other restrictions are determined by the geographic region to which your Apple Account is associated.
As such, an iPhone purchased from a UK retailer may always have any UK Age Restrictions and associated Age Verification - regardless of by who or where the device is ultimately used.
Age requirements for managing an Apple Account in the UK - Apple Support
If you're asked to confirm that you’re an adult - Apple Support