I have taken my two boys (now 8 & 6) to the Uk at least once a year, since the eldest was 15 months old (when we moved here) - and often alone! The flight is 7 hours (plus 3 hrs at airport & 3 hrs by car other end, oh & one way is overnight!).
I have NEVER given them medication. I firmly believe that anyone who needs to drug their child on a plane should not be taking their child on a plane! I'll probably get flamed for that but.......
Yes, it is hard work keeping them amused - but not impossible. I survived the journey with a 4 month old & 2 1/2 year old both ways without DH!
Yoiu just need to be prepared, as the OP is trying to be. A few new surprises mixed with old faithfuls always works. Try not to take anything too noisy. Colour Wonder products are great for planes. Snacks are good too. Don't forget the restrictions on liquids! I would take an EMPTY sippy cup & ask the stewd(ess) to fill before departure (unless you are able to buy drinks after security & board the plane with them).