Originally Posted by
orbitmic
My understanding is that BA would assign Ms XCstud and toddler seats together ...
Doesn't this depend on the age(s) of the child(ren)? The OP is ambiguous because it refers to a £60 saving (implying three passengers on HBO fares), "my wife and daughter" and "we have a toddler and infant".
If the HBO booking includes an infant, then IIRC that does include seat selection by the adult passenger. But if the HBO booking is adult and child only, then BA allocates the seats shortly before the flight.