First, you complete situational and numerical tests. The second stage is a video interview where they give you unlimited time to answer three pre-recorded questions.
If you are successful, you are invited to a virtual assessment centre. Here, you carry out a group exercise, a meeting simulation (where you role-play presenting information to a department head) and a 1 to 1 interview. I personally found the interviewers to be unsociable and not as friendly as an interviewee would hope, seeming unable to crack a smile, which made it difficult for me since I, as a candidate, was trying to come across as friendly. They seemed uninterested in the interview and in getting to know me as a candidate, bringing conversations about professional life at BA to a halt and reluctantly answering basic post-interview questions about the job. I hope they are easier to work with than it seems!
Advice I would give to those about to do an assessment centre is to prepare for many situational questions, as well as to do lots of research on industry trends. I was asked around 10-15 questions just about times when I have shown certain skills, which proved difficult as some of them were similar and I did not want to use the same scenario more than once.