We flew last year when my older son was 13 months old and just learning to walk. This time when we move to the UK we will also have my younger son which will be 12 months when we fly. We usually fly BA direct from BWI (Baltimore) to Heathrow. BA is usually VERY helpful. One suggestion if you are up to it, is to get there AS SOON AS check in opens for your flight. If you get there first, it is more likely that they will have a bulkhead seat open, and when you have a young child or infant, they always do all they can to put you in bulk head. (on our way back we actually got upgraded in order to be in bulkhead). The main reason for this is that we did not get him a seat as he was under 2 and not required to have one, and after take off, they have baby cots that strap to the tray table in bulkhead. It was great. My son fell asleep on take off (thanks to the help of the night flight and a little benedryl), and then we stuck him in the cot and he didn't wake up until right before breakfast was served. It was a little difficult as we had another screaming child next to us, but it wasn't too bad as my son was a deep sleeper. This time will be good because we have to get a seat for our older one and so if we are placed in the middle, we will have all 3 seats! We are actually flying Virgin Atlantic out of Boston this time. I have heard that they are very good with kids and actually on their web site, they say that they have nintendo games that they can play right from their personal screen! I am looking forward to this.
Definatley take the umbrella stroller. I have roamers, and it helps to get them calmed down if they aren't allowed to run around before hand... of course, you may choose to want to let them run it out outside the plane while waiting to work out all the energy and get tired out while on the plane, I guess it depends on the child!
Backpacks are wonderful! I never use anything else when flying for my carryon luggage.
One tip.... a lot of airlines have lowered their baggage allowance for both checked luggage and for carry ons. Check this! you don't want to be caught with too much stuff.... A good tip though is that if you read everything about luggage, anything that is NESSASARY to care for the baby (diapers, wipes, food, bottles) do not count toward your carry on luggage allowance. You may want to pack all this stuff in a separate bag from things like toys and such, that way it is easy to grab for changes and you can tell them that that is what it is and it doesn't count towards your allowance.
Another tip for toys... make sure there are not lots of little parts...... if there is, maybe tie them with strings. We ended up loosing quite a few toy cars while on the plane as he would drop them and they just roll.
One more tip.... if you do plan to take a car seat..... There are 2 options that help for this as far as portablity.... There is the wonderful Sit 'N Stroll... It costs about $200 dollars, but I have heard from people that have it, it is very worth it! Another option... if you own an american Britax car seat http://www.gogobabyz.com/products/gogo_kids.html
This just attaches right to your car seat and turns it into a stroller type thing like the Sit N Stroll. I wish I had the right kind of car seat in order to get one!
Sorry to be so long winded!