Flying from DFW to LHR was my 2 year olds first long flight. (Cuba wasn’t that long) One thing that really helped me out was leaving at night time. Our flight left at 7:10 pm which is 1.5 hours before Carson’s normal bedtime.
The day of our flight I had my son take a early nap. (So that he would be ready for bed time earlier) Once he woke up I took him to the park to have him burn off some energy, gave him a bath, then it was time to head to the airport.
While at the airport, I let him push his suitcase anywhere he wanted to tire him out before boarding. He was sleep within 30 minutes of take off and slept the entire time.
Tools: I bought him a blow up leg rest off Amazon which allowed him to have space to kick his legs up and be comfortable during the flight. You are able to use these on the plane as long as it’s not in an aisle seat.
Most international planes offer free in-flight entertainment, but I would also recommend downloading some of your child’s favorite shows on Hulu and Netflix. (Show will play with out WiFi)
Time Change: The time change didn’t effect him because with us leaving at night, we arrived in London at 9am so he woke up ready for breakfast and to start his day.