MH is not the only airline that can get passengers between London and Kuala Lumpur although it is the only direct option. Having spoken to a friend who is MH crew their flights between KUL and LHR usually have few if any spare seats so it's probably not surprising MH are reluctant to accommodate a goodly number of tickets from BA. BA will be talking to other airlines both within OneWorld and others. Patience is required.

And under EU261 they cannot fly you to Singapore and wash their hands of the situation under a commercial rebooking policy. But those who have rebooked to SIN have probably done so under the standard rebooking guidelines using the passenger voluntary option to shift to an airport within 300miles and have probably used their one free change combined with a voluntary change of KUL to SIN airport and probably have no recourse against BA to get them to KUL any more. But that's just an clueful guess on my part.