hi Ali--
Your trip sounds fun! The most important thing to keep in mind is that whatever happens on the plane, you will eventually be able to get off!
The trip will end at some point and then you will have a chance to get back on track. We have made a few trips of this length with our two kids, and this is the mantra that I sometimes repeat to myself.
As for how to navigate the whole leaving and airport thing, I think some of it depends on the temperament of your baby. If you put her to bed and woke her an hour and a half later, would she be fussy and irritable at being woken up, or would it serve as a way to recharge her to get through all the airport stuff? My first thought was to go on and put her down at the normal time, maybe with a shortened routine, partly to get her a little catnap and to give yourself the time you may need to do last minute packing and preparations. If you think however that it would make things worse to wake her, then keep her up and hope she falls asleep at the airport.
I have to say in our experience though, that activity at the airport has usually been too exciting for the kids to sleep through. Plus you will have to take her out of the stroller to go through security which may wake her. We've found that the kids usually don't drop off till we are on the plane and basically settled--too much noise and activity before then. But then, they are more sensitive sleepers. That's why I'm wondering if a little snooze at bedtime will help her get through all the airport stuff and then she'll sleep on the plane.
The last time we did this, our little boy (14 mths at the time) fell asleep in the car on the way (we left around his bedtime), woke when we got to the airport (about 45 min to an hour later) and was awake till we'd been in the air for a bit. It was by no means ideal, but I'm not sure there is an ideal solution!
Let this comfort you though, you already have her on a really good routine, so she will bounce back really well when you get to London.
Also, I'm sure you've thought of this, but be sure to take a few familiar sleep-time things with you on the plane--blanket/lovey. That helps alot. And I know for me, I've had to redefine my expectations from what I expect on a "normal" night. In that, I consider the trip a success if the baby isn't crying or fussing alot and gets "some" sleep. If she ends up awake more than you'd like, but relatively content, that's actually pretty good! On our last trip, i spent a lot of time holding our ds and swaying in the aisle more that I would've liked, BUT he slept in my arms and didn't scream and wake everyone. So, I kept telling myself that it was working out okay.
Best of luck to you!! Let me know if you have any more questions...I am by no means an expert, but have definitely been in your shoes!