Hi - I'd like to book LHR>SAN>SFO with Avios and no stopover and legal connections, 2 people, business on BA and First on Alaska.

I can't do this online, so have called in twice and taxes are approx $1900USD total for the trip. If I look at each segment individually online, taxes for LHR>SFO are $900, and SAN>SFO (on Alaska) are $12.

Is there anything I should be saying to the BA agent on the phone to get the lower total tax amount? They just tell me it is what it is with varying reasons that are inconsistent. Thanks.