Either flight you are pushing your body to stay up later. In the case of the morning flight, I find it easier because by arrival time it is afternoon in LHR and late evening in SIN. By the time you get to your hotel, get cleaned up, have a bite etc. you have just to occupied yourself for a few more hours and you can, I find with ease, push your body clock a few hours past midnight SIN time before going to bed. And of course, best part is not having to wait for your hotel room to be ready. When I have done this timing to Europe, I have almost no jet lag.