BA does have SIN based crew. They fly SIN, SYD, PEK, PVG and the new CTU as far as I know.

They also often do "V' routes, so SIN > LHR > PEK > LHR > SIN.

There are at least 1 but usually 2 to 3 of them per flight - reason is to have a local language speaking crew presence especially for emergency purposes.