If that was the case they'd have been selling F out of LGW too.... some of the ANU rotations have been operated on 4 class triples for years. J/C fare buckets can also be more lucrative than A fare bucket. Also no compensation to pay out in the event of an aircraft swap to a 3 class. Catering out of ANU is also pretty horrific so perhaps there are operational constraints which prevent a long term F service being provided. It's pretty unlikely they're reserving F for a positioning crew - if they're allowing foreign nationals to enter for a holiday then there's no reason the crew wouldn't be permitted to do so either.