If you can book with British Airways you'll be fine with infant ticket - and will even get a free seat assignment for the return trip, when she is over 2.
Yes! Just happened to me. Every airline requires that you have a real seat for him on your way back, but only British airlines gives it for the same baby rate as if he was under two.
Just one thing to keep in mind - on the at back, make sure you get to the airport early enough. Basically, we got to the airport 2 hrs earlier and even through I had the eticket for my baby, it wasn't ticketed or something. They wound up spending about an hour fixing the seat and we missed the flight. They actually put us on a direct flight with elal because of the inconvenience
Good luck!