How then are Oneworld fliers supposed to travel on BA on mixed carrier itineraries with their sporting equipment when it will invariably bring them over their 20Kg allowance?
Kingbat, you are correct. Basically, anyone with sporting equipment - be it skis, golf clubs - will definitely run into the same problem with mixed carrier itineraries on OneWorld.
The thing is that I regularly travel - HKG-LHR-GVA-LHR-HKG on CX & BA and HKG-LAX-DEN-LAX-HKG - with ski equipment. Each bag weighs at least 20Kg. I am even above my gold card allowance. I have never been charged to the extent ewong has been charged. On my upcoming ski trip, I'll leave enough time to re-check bags at LHR!