Hi Exezy,
The BA website and a quick google should give you what you need to know. They are looking to see that you have a genuine interest in the company and have bothered to do some research. As such you are likely to be asked what types of aircraft BA operates but less likely to be asked how many. (And giving registration letters will almost certainly be a fail point
)
Topics that came up for me were: T5, and open skies. You can add to that list, of course, the recent aircraft orders.
The shareholders section of ba.com has a podcast of the latest shareholders meeting and this will go a long way to preparing you.
Good luck!
KT