I think others will be able to give you better info, but just to get the ball rolling, I would conside looking at a Britax Roundabout, Cosco Scenera, or Evenflo Maestro. These are all a little bit slimmer and lighter than the MyRide.
We have used a Roundabout many times and have never had problems installing it on various aircraft of different sizes/configurations.
The Maestro is less expensive and a bit lighter, and as a combination seat that converts to a booster, it may last you longer. The Scenera is the lightest and least expensive of the 3, but (I think) would be outgrown relatively quickly, possibly already outgrown if your child has a long torso.
Would this carseat be just for the plane ride or also for use in a car once you're in the UK? I might consider something with lock-offs (e.g. Roundabout) unless you know that the vehicle you'll be using has Isofix (LATCH) or has locking seatbelts. Otherwise you will need a locking clip, which I personally find hard to work with.
Also, which airline is this? As you probably know, some of the non-US carriers have restrictions on carseat use that may limit your options further. If there is a codeshare, the policies that govern are the ones of the airline that is actually operating the service.
Finally, if your airline permits the use of CARES and you think your daughter would be comfortable in the airplane seat without the support of a carseat, you might consider CARES ($75) which attaches to the lap belt. I've never used it and people here seem to have mixed opinions.