OK. I have no legal qualification. I can only tell you what I do with my dual passports and what I have done for several years. When I leave Australia, I use my Australian passport and I fill in my APIS form, which is taken at the airport, using my Australian passport details. When I enter the UK, I show my UK passport at Customs LHR but I don't have to hand in an APIS. Neither does my wife who uses an Australian passport with Riight of Abode. As far as I know, an APIS is only needed if a non EU passport is being used. When I return home, I use my Australian passport and I fill in the APIS to show that I am an Australian Resident returning to Australia. As far as I know, this is perfectly legal if you hold dual passports.