It will be similar to the following. Two interviers one from flight ops and one from people department and the questions will be much the same as any other Corporate style. Experiences, what can you bring to the company, you may be asked about your knowledge of Oneworld, its members and its goals. Also be on the ball with BA's corporate / environment / good neighbour concepts which should be on the web site. Be aware of size of fleet / income earned / general knowledge of the industry and problems at present.
On arrival you should make sure you read the weekly 'BA news' and get a feel for what has happened this week. The interbviwers will mark on several key areas; Attitude / ambition / avaition knowledge and business knowledge. One will be writing all the time. Make sure you answer the questions acurately and stay focused. If you are not sure or do not know say so or ask the for the question again. it is also good to show flexibility (an example) and an idea of your goals and career targets, without being over confident.
They will mark you as well on how attentive you are and your manner. Sit upright , lean forward and make surebthey see you are really up for it and totally focused. If you get a chance look up some of these issues and get some one to interview and get them to comment on your style and possible improvments.
That said you will fine it very relaxed and friendly. Be suited booted and a business tie (not to loud). Good luck.