Flown to kuala lumpur (KL) Malaysia a few times, a lot depends what time your flying out from the UK, a midday departure is nice as it gets you into Malaysia at the start of their working day, so plan your sleep time on the plane as late as possible, or if leaving the UK on a late flight ie around 22:00 to 24:00 plan your sleep early on the flight as that gets you into Malaysia mid to late afternoon, too late to do any work, so you can relax, check in, enjoy your evening and be fresh for the next day.
Coming back works the other way round, ie, leave Malaysia on a late evening flight gets you back to the UK at the start of our working day.
I would drink, but not too much, and drink plenty of water or juice on the plane. another trick is to set your watch for the destination time as soon as you get on the plane, that way your preparing the brain on local times.
Ive flown Qatar Airways, nothing like Ryan Air, their good, good food and good service, one of the better ones a lot better than Malaysian Air. Doha is about half way so its a good stop to stretch your legs for an hour or two and breaks up the journey, be prepared to pay £5, $5 or 5 Euros for a cup of Coffee though.
Not sure where you going to guessing KL, but a must is to eat out in the streets at least once rather than the Hotel, try a get some locals to take you out if possible, or venture out on your own, its a great atmosphere at night.
Might as well start saving your Air Miles from the first flight, as you will be wanting to go back there one day, good luck and enjoy your free time.
Chris.

Edit: about 12 Hours coming back, from memory.