If you want to book it on one ticket - which I'd recommend - you have to find an airline which publishes a fare LON-PPT. There are several options, however, neither AA nor BA is amongst them.
As far as Oneworld carriers are concerned, there is LA, QF and QR. QF is routing via SYD or MEL; QR via DOH/AKL, using NZ for the AKL-PPT leg. LA is either going through SCL-IPC, or - rather surprisingly - also via AKL using TN for the AKL-PPT leg. This offers some flexibility, as compared to the once a week flight via IPC.
Obviously, there are options with other alliances, such as AF/KL (via LAX) or NZ (via AKL) or EK (via DXB/AKL using TN on the last leg).