I presume from what has been said that a separately ticketed Oneworld Visit Europe ticket which must be cross-referenced to your ticket taking you to Europe should be exempt from the APD if the arrival in LHR occurs less than 24 hours prior to the departure on the 2nd ticket.

While I don't have any experience with this situation, I also presume that you would have to go to hell and back in your attempts to get the APD waived on the 2nd ticket.