The wording of this question seems to suggest that the title Marquess of Anglesey indicates some sort of suzerainty over the island of Anglesey. The truth (unless I'm mistaken) is that when granted any sort of peerage, the subject simply chooses the name of some place with which he or she wishes to be associated. Paget (who was already the 1st Earl of Uxbridge) chose the title Marquess of Anglesey because that's where he had his country house, Plas Newydd.
© Macclesfield Quiz League 2019