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You must get the permission of everyone with parental responsibility for a child or from a court before taking the child abroad. Typically a letter from the person(s) with parental responsibility for the child will suffice and should include the other personβs contact details and details about the trip.
Source: https://www.gov.uk/permission-take-child-abroad
See also https://www.airbaltic.com/en/children-travelling-alone βchildren of UK citizens need parental permission to travel independently or accompanied by an authorized personβ