Is a perfectly valid Oneworld Explorer. So the BA agent you dealt with was certainly incorrect.
TBH - I wouldn't include LHR-DUB-LHR in a premium cabin Explorer.
And also I would fly CPT-LHR at the start, unless I had a reason to go to JNB at the beginning as well as at the end.
Then I'd use the segments freed up to fly some decent premium cabin routes (for example LHR-DOH-LHR in QR's A380)