Buy from: Norwegian.com
Best Prices (Nett)(Return/PP)
Singapore -> London -> Singapore
USD 448 ~ IDR 6.200.000
This is a budget airlines so baggage check-in is an extra charge and be sure to bring your own meal as the meal is an extra charge. The good thing about this flight is that it is a direct flight from Singapore to London, so you only need to transit once in Singapore if you are flying from Jakarta.
Holiday Mulu Opinion:
Even though this is not the most comfortable option to go to London, this is the cheapest option. Even adding a [budget] ticket from Indonesia to Singapore would still be quite cheap. It is more suitable for people who wants to have a quick getaway without any check-in luggage as it is more worth the money to get a more expensive airlines with check-in luggage and meals!