Need help from the experienced people here.

I am looking at booking flights for a 60th birthday trip in December for my wife of 36 years.

I want to specifically route via London so that we can do some quick shopping and hit a favorite restaurant before going on to Tel Aviv.

Problem: when I go to the "Multi City" booking and put in MSP-LHR, LHR-TLV, TLV-LHR, LHR-MSP; it gives me the "sorry you can't do individual segments to do a multi city booking.

I can do this on the BA.com website but I lose free seat selection (Life time Platinum on AA, Blue on BA).

Any tips on how I can beat the AA web site?

Thanks John