I believe that because Virgin Atlantic has a code-share agreement with Malaysian Airlines, VS flights stop over in KUL instead of SIN on their way to Australia. Mind you, that's not bad because KUL just recently opened a very nice, brand-new airport.
Virgin Atlantic always had plans to fly into Singapore and on to Australia to compete directly with BA and QF since the airline was conceived. The UK CAA however has never granted VS permission to operate on the UK-Singapore route....(nb...the UK CAA does still implement archaeic laws on routes).
Malaysia Airlines meanwhile was getting fed up with its constantly full 747 flights which were flying into LHR once daily during the late 1980s...(the CAA restricted MAS' services in the UK and still does) so Virgin and MAS struck a deal where they would code-share on the LHR-KUL route. With this deal - MAS was able to double its services to London as VS at that time did not have spare equipment to fly into KUL itself.
On the Australian side - they were happy that VS would not be encroaching on QFs hub at Singapore and therefore granted VS rights to code-share into Oz with MAS. The benefit to VS would be that after 10 years of code-sharing the Aust. govt. would grant VS their own direct services into London.
Since then - the MAS-Virgin code-share has been pretty successful with MAS requesting a third daily service into LHR - which has been rejected by the UK CAA - surprise! surprise!
VS meanwhile has been bought up by SIA and QF is now about to buy MAS - so how the tables have turned!
I reckon by 2004 - when the VS/MH code-share deal runs out - we will see VS deserting KUL for SIN instead. VS had promised to start its own KUL service but passengers are still waiting and that day will never come. However, both MAS and VS have both gained substantially from their code-share even if they both go their separate ways.