Hi,
I am in Orlando just now, flew out with BA. As 2Tiggies said it is 28 pound per person to pre book but if you do on line check in 24 hours in advance of flight it is free to choose seats. Also if travelling with children under 12 they will allocate you seats 5 days in advance but you are also free to change them at on line check in. I am a nervous flyer and wondered whether to pay for peace of mind but there was plenty of seats to choose from when I checked in on line. I am glad I didn't pay the money.
Rach x