I would certainly recommend sleeping pills as a means of ensuring a decent night's rest on a long-haul flight, whichever cabin you are in. However you will not find any pharmaceutical compounds (as opposed to natural "sleep aids") available over the counter in the UK or, I think, anywhere in Europe. The good news is that many GPs are happy to prescribe them precisely for use when travelling or to counter the effects of jet lag. It will probably be easier if you go to your regular GP though I have had them prescribed also through a private "drop-in" medical centre with no problem.