GG, Being a resident would be sufficient under UK law. More interesting however is whether Delta actually knows what country you are a citizen of. Your Skymiles info is linked to your residency, not citizenship, and although they may take Passport info before a flight, this would not take into account whether you had dual citizenship (remember you do not have to have a second passport to be considered a citizen of a second country, dual citizenship may be based on marraige, place of birth, parentage, etc. - just refer to the info pages of your US passport for the verbiage), or whether you are a resident of another country.