Can someone please help me understand baggage allowances?

I am WP and I have a RTW ticket booked in PE where I can, but Y on a few legs within the US because there is no PE.

I booked the ticket directly through QF, but because of the routing, some legs are with QF, some are with BA and some are with AA

So, I have one checked bag that weighs about 30kgs, plus normal hand luggage.

I have no problems at all getting from SYD-LHR, but when I checked in for LHR-JFK with BA, they charged me £30 for excess baggage??? I said to the woman that this couldn't be right because surely as a OW Emerald in PE I would be able to have extra baggage, but she was adamant that I could only have 23kgs...

Is this right and am I going to have the same problem in the US when flying AA??

Appreciate any assistance!