aaaaa: Hoewever most people say they need an average of eight hours sleep a night, so that still leaves a few hours or so when the blinds could be opened. Personally, I don't sleep on planes at all well, thus I admit to having a vested interest in open blinds.
A LHR-HKG will only allow about 8 hours of sleep at best, anyway, except for the most dedicated sleeper. The actual flying time is something like 11 hours. It takes about two hours to serve and clear away dinner, and then the before arrival annoucements start about an hour out - the second meal may be served a bit before that. So even your suggestion would have the blinds left down for the whole of the cruise portion of the flight.
Most LHR-HKG flights are mostly in daylight, particularly during the northern summer, and especially if you are on the flights that depart after it has already got dark in London. On the late flights, for example (c.2230 departure), it will only remain dark for three or four hours, and then the remainder of the flight will be in daylight.
But (as has been said) you don't have to worry about the Sleeper Service on the HKG route - you won't get it. But you may entertain yourself by seeing how many of your fellow passengers adopt a self-made version of it by getting on board and going straight to sleep, particularly in the HKG-LHR direction.
Me, I'm looking forward to a long kip on a LHR-HKG tonight ... desperately need the beauty sleep.
BRUpax: Most FAs answer with the short night bla bla, but fail to answer the point raised about eye masks. What are they provided for if not to hide daylight?
Eyeshades are not 100% effective. They work better when the cabin is dark to start off with.