Hi
jamesegrant. The definitive answer to this one is that under oneworld rules you and your son would have access to the GF lounge regardless of status having arrived in F and connecting to a domestic flight. The basis for this can be found on teh oneworld site and also
here on the BA site under lounge access:
Connecting between oneworld marketed and operated flights:
- First and Business Class customers connecting on the same day of travel, or before 6am the following day, can access the lounge when travelling between an international long haul* and an international short haul or domestic flight (and vice-versa). (*a oneworld international long haul flight is defined as an international flight marketed and operated by any oneworld carrier with a scheduled flight time longer than 5 hours)
- Lounge access will be determined on the international long haul ticketed flight (either First of Business Class) regardless of the ticketed class of travel on the international short haul or domestic flight.
- You must be prepared to show your boarding pass or itinerary showing travel in First or Business class on the international long haul flight, in order to access the lounge before your international short haul or domestic flight.
In addition to that, there is the CCR which not a oneworld lounge so the oneworld rules don't apply. However, the BA rules state that when coming in on BA F and connecting on to a domestic flight you have CCR access. [EDIT: see
edi-traveller post above and the scan which shows that access to the CCR would be allowed]
Whether GF or CCR you would have to show your inbound F bp when trying to access the lounge.
Certainly the reason your son was given would be incorrect in your situation. Which lounge did he go to? Did he mention he had arrived in F?