The following does not constitute medical advice.
I find diphenhydramine hydrochloride is useful in my travels for when I absolutely must get some sleep before arrival. It is an antihistamine usually sold under a trade name such as "Nytol" (UK) or "Benadril" (South Africa, USA). In my experience it plays well with food and the moderate quantity of alcohol that attends any dinner (although excessive alcohol consumption should be avoided). It also has a relatively short biological half life so I do wake feeling quite refreshed. However on the eastbound transatlantic you will struggle to get in more than six hours so you may be drowsy for a few hours after landing.
The only side effect is that, as an antihistamine it does leave your mouth very dry, and this is redoubled by the effect of the low humidity air onboard airliners. Have plenty of water ready for when you wake up. There is also a mild dependence aspect from repeated use but this should not be a problem unless you are going to be shuttling between the USA and Europe several times a week.
Obviously, read the packaging carefully and if necessary consult with your doctor.
Aside from this, my advice is eat in the lounge or the terminal well before boarding, so once on board you can lay out and fall asleep shortly after take off. You will see the weekly commuters on the NYC-LON route are asleep before take off and only wake on landing. Use the provided ear plugs and blindfolds too, even if they feel uncomfortable at first.