The information that we have, and I'm sure that you will have been told by BA is that both the traditional airline bassinets and Britax seats' are allocated on check in, so do CHECK IN EARLY.

We had heard that you only get the option of a Britax seat if your infant has a seat of their own booked, so if this is not your experience, please do let us know.

Generally, parents prefer the Britax seat, and say it is much better, but also say that when the seat belt lights are on, you have to take your child out of them.

This is information from British Airways regarding infant seats :

"British Airways is the first airline to offer a specifically designed seat for infants/babies. Designed by Britax, the seat is ideal from 6 months to 2 years, but could also be used from birth. The seat can be used in a variety of positions from horizontal to vertical.
The maximum allowed weight of the child in Britax seats is 13kgs/29lbs.
Britax infant seats are not pre-bookable, they are allocated on a first come first served basis at the airport or onboard the aircraft.
Carrycots and infant seats are available free of charge on all longhaul flights, except in First class on Boeing 777/767 aircrafts where only infant cots are available.
These seats are normally located at the front of each cabin section (the bulkhead position), therefore when you make your booking please request a bulkhead seat to ensure the carrycot position.
Currently bulkhead seats cannot be requested online, therefore to request a bulkhead seat to ensure the carrycot position, please contact your local British Airways sales office."

For an image and review of a Britax seat, follow this link to Daddy Types, the weblog for new dads

We hope this answers some of your questions. Good luck and happy holidays!