It annoys me how people often assume you have the option to take a car / train / holiday in the UK instead. A lot of us do not have family here, so yes, the option exists to only holiday in the UK - and never see grandparents, cousins, uncles and aunts. I don't think that's very healthy either.
That aside, I wouldn't drug because ds reacts differently to the same medicines at times. I have flown with him on meds (got ill on holiday and had to take them, so no, I didn't have the choice not to fly) before, and yes, he slept for 8 hours straight, which he never does at home. I was very worried as he was also very pale, as another poster mentioned earlier. At the same time, I have given him allergy medicine at home (consultant paediatrician advice for something else), and he didn't sleep longer than 2 hours at a time. It did help for the allergy though. At other times, he slept well on it.
So, in summary - no, I wouldn't drug on flight as too risky (for me). I have no problem with someone else giving something mild though if they know how their children constantly reacts to it. I was shocked at stories my mom told us of people in the 70's drugging their children to sleep in the cars whilst they partied.
(Mom never did, for what it's worth, but then, we all react bad to medicines, so it would have been stupid anyway.)