Fly from LHR-KUL (London-Kuala Lumpur) and stop-over, option to take a train from KL to Singapore, return,

Fly KUL-DPS (Kuala Lumpur to Denpasar, Bali)

Travel overland from Bali to Jakarta

Fly JKT-LHR (Jakarta-Heathrow)

All the above flights are available with the superb Etihad Airlines, for under £1,100