I used to fly with BA, LHR- CGK via KUL.The first leg was always quiet full whereas KUL-CGK would sometimes have more crew than pax. There does seem to be very low demand even now for a London – Jakarta service.
A way round it would be to go via the Middle East. Everytime I fly Emirates the DXB-CGK flight is rammed with migrant Indonesian workers returning home.
The Garuda flight was LGW-BKK-DPS and was often very full, but sometimes quiet empty. The nail in the coffin was the Bali bombing which for a couple of years reduced Bali to a ghost island. The last time I flew with them two of us had the upper deck to ourselves both out and back.
Edit to add, the upper deck was P/E. £125 extra each way, sometimes with, it seemed, unlimited champagne.