M,
I had the 'pleasure' of going through the final board today. The interview was probably the toughest part as you do get a good grilling. The interview is done by a pilot and an HR person. They're both very relaxed and friendly.
Most of the questions are about you. They're looking for reasons why you have made the decisions you did in your life, examples of the things they're looking for (further examples of the sections on the application form). Interestingly, none of the guys doing interviews today had much on aircraft tech or on the BA business.
I'd recommend you concentrate your preparation on answers to questions about you, rather than tons of details on British Airways, their fleet or aircraft.
Good luck