Hi Claire
I've just returned from a 3 week trip to the UK, my son is also 4 months. Just try and stick to your normal routine. My son slept the whole way was fed on the way up and then a couple of hours before landing. He had some water when we were landing for his ears (which only lasted 5 mins).
For the flight have a small seperate change bag/ wash bag with nappies etc, so you can tuck it under your legs, this means you wont have to keep on getting the bigger bag from the over head locker. If you are flying BA request row 40, the ailse seats, this will mean you and your husband will only need to disturb each other during the flight and not have to jump over another passenger.
with regards to your buggy, you can push it all the way to the gate and they should store it in the plane, in my case they found a cupboard for it.
I also completly over packed, all you will need is a couple of spare baby grows and about 6/7 nappies. How many times do you change bum in middle of night at home?? once maybe??? I took about 15, which was a complete waste. Just bring what you would have in your change bag and times by 3 and maybe some calpol to be on the safe side.
Good luck, its not bad at all, I had the best sleep on both the flights, better then when i travelled without baby.