Originally Posted by
Cymro
In that case, as it is not on AA (or OneWorld) metal you would need a BA codeshare - and while AA codeshare on that route, BA don't. Unfortunately, you've now found the limit of where OneWorld ends.
To elaborate, the fact that BA doesn't codeshare on the route isn't the problem in terms of
booking it via BA. The question there is: do BA file (reasonably priced) fares either through from LON to PPT, or LON-LAX BA fares combinable with LAX-PPT AA or TN fares. If so, then BA would in fact be able to sell you the ticket - you just wouldn't earn avios/TPs on the LAX-PPT leg.
In other words, codeshare is only necessary for avios/TP purposes, not actually for ticketing purposes.