since nobody else mentioned it...

your seat (ORD-LHR) will have a fresh pillow and duvet/blanket, good for sleeping. You'll also have an amenity kit at your seat that contains a toothbrush, toothpaste, mints, moisturizer, comb, socks, eye shades, lip balm, pen, ear plugs, tissues. You're welcome to take the amenity kit home with you.

Your seat also has a DC power port. Get an adapter/inverter if you need power for your laptop/device.

my best advice is that if you're hungry, ask for seconds. When they come up the aisle with the hot fudge sundae cart, they ask if you want a sundae or a fruit/cheese plate. Get both if you want.