In that case I'd definitely try for *A. The economy booking class higher (thus more likely to be available).

Something like LHR-JNB//JNB-GIG//MEX-LAX-NRT-PEK//PEK-BKK-KTM//KTM-BKK-LHR is just under 34000 miles according to great circle mapper, thus a YRWSTAR2.

Note I haven't checked any availability - this may require rerouting and bump over 34k threshold into a YRWSTAR3.