Found some seats that MAY fit:
SunshineKids Radian car seat. weighs a mere 30lbs, but it folds to 7inches!
Safety 1st onSide Air.
And Britax Prince.
Down side is I now discover that USA car seats are not usable in the UK...seatbelts in cars are different!?
Looks like I'll have to travel out with 2.
AmericanAirlines op allow infant seat and rear-facing seats so will bring that, which is also compatable with UK seatbelts for in-car use.
return to usa with BA will require (for us) bristish version of above seats where possible, that we might use them in the future too.
THANK you all for your assistance.
I will keep on til I find the answer that works for our daughter...God, I'm so lucky to be able to write that!!