Agree with the above re. Air Asia X's fares not being attractive enough Long-Haul.
The cheapest I've ever seen Air Asia X on LON-KUL return is in the region of £450-500. Last week I managed to get Emirates LHR-BKK via DXB for £503 return, 3 of those flights on brand new A380's with top class entertainment. Not to mention that you get more than enough free luggage allowance, unlimited free food/drinks onboard, and all of the other perks such as seating requests and online check-in.
Short haul is another matter of course - I booked SDK-BKI (Borneo domestic flight) for £16 including pre-paid baggage and seat selection, which is a bargain!