I share your preference for Popeye's over KFC. And I completely understand how having some familiar fast food (especially Popeye's chicken!) would make you very happy. Now I'm thinking about it... *wistful* It's way too far to drive just for some chicken, though.
Unfortunately, access to RAF Lakenheath is controlled (some bases are open, or at least they used to be... security may be different these days). Since you are no longer a dependent, you would need to know someone who has base privileges (active duty military, retirees, dependents, and government civilians with a valid military ID card), and they would need to either sponsor you on, or go buy some Popeye's and bring it off base for you. But if you want to contact the RAF Lakenheath Visitor's Control Center, you can phone them on 01638 523 554/560, in case they have different information.