You can pre-order formula at Heathrow (Reserve & Collect), and baby food if you are at that stage, and collect it from Boots Airside. It will save you the trouble of getting it through security and it will be there, ready and waiting for you. I think it may also be slightly cheaper than buying it landside. And get more than you think you might need. Worst that happens is you waste a pound. You can abandon it on the plane if you don't want to take it with you.
Try to feed your DS at take-off and landing. It will help with the change of altitude. Dummy if he takes them and isn't hungry. One of your fingers if he doesn't like dummies.
If naptime is near to your flight time and DS is agreeable to having naptime shifted a bit, see if you can keep him awake until you are on the plane, then let him pass out once in the air (and in a skycot if possible). It's not worth it if he will be hysterical, though.
BA also do bouncy chairs (similar to this) in the bulkhead seats, in addition to the bassinets, which might be better for a 4mo. Ask for it as soon as you get on board and stake your claim - they may not have enough for every bulkhead seat and they are definitely worth having.
Pack every set of spare clothing (including spare clothes for you) into their own plastic bags. Then when you need to change clothes, you have a plastic bag handy to put the soiled ones into.
Don't take DS out of the skycot automatically when the seatbelt sign turns on if it's not noticeably bumpy. Sometimes they turn it on when they just want everyone to sit down (usually when they are coming through with trolleys) and don't mind if the babies aren't in your lap. Depends on the flight crew and always be pleasant with them. They can make things much easier for you.
Take at least twice as many nappies as you think you could possibly ever use. Change nappies just before boarding, whether or not he needs it.
Don't get stressed (if possible) if the baby is crying. Other passengers might not like it (they won't, but some are spectacularly sympathetic), but getting stressed about their reactions is only going to make you tense, which your DS will pick up on.
If he is nicely settled in the skycot, leave him alone! Read a book, watch a film, SLEEP! I was once on a flight where the mother and grandmother fussed and fussed over their LO, who was fine, happy, sleeping, and was fussed at into being fussy. If they had left her alone until she started fussing, they all would have enjoyed the flight more.
Give yourself lots of time. If you think you are running late for your flight once in the airport, enlist help from airport staff. Tell them what flight you're on, get them to tell the crew you are on your way, and see if you can get in one of those cars that people with mobility problems get ferried around in.
Get on the plane early and see if there is a friendly flight attendant who would love to cuddle a baby while you arrange your things. Get off last and take your time to get sorted. Always take help when offered. If it's not offered, ASK.
Take a baby carrier with you onto the plane, just in case you don't get your pushchair back at the gate. You probably will, and you probably should, but you don't want to find out after 8 hours flying that they have sent it to baggage claim and then have to carry a baby and all of your things. Actually, if you put DS in the carrier in the airport, you can use your pushchair to carry all your hand luggage. 
I'm jealous, if you can believe it. We are going to the US on Saturday with two very active toddlers. It was much easier when they were babies!