Congratulations to those who made it, you have been selected from a large number of applicants and obviously had what BA are looking for. The hard work starts now, there are 14 ATPL exams to pass along with a number of demanding flight tests before reaching the ultimate goal. My advice would be to take each stage at a time, don't treat each stage as a box ticking exercise but rather add it to your "tool box" and use the subsequent stages to build upon it. At times the course will be tough and it's no secret that the courses that bond together as a group the best achieve the best results, the course will give you some excellent new friends along with some brilliant memories.
For those that weren't successful. I'm sorry to hear that, I know it's unpleasant to find out and puts a total downer on the day. I recommend a few days away from anything aviation related and just get out there and do some activities you enjoy. What is important moving on from here is adopting the correct attitude, I know it's natural to apportion blame elsewhere but it's so important to complete a thorough self-analysis of your performance at every stage and try to work out what you can improve on for the future. Luckily BA have made this fairly easy this year being that the final stage is the interview so this process becomes far easier!
I'm unsure if BA provide feedback? I would certainly recommend giving them a call to find out. Think about the questions you were asked? Do you feel you conveyed everything that you wanted to? It's well worth starting to document everything you do in both your professional and personal life that could be used to answer a competency based interview question, very soon you will have a rather large document and it makes preparation far easier. Like with anything a healthy dose of luck is required, it may be on this day you rolled a one but could just have easily rolled a six. However, what isn't based on luck is how you use the time in between application periods to improve yourself to increase the chances for next time. For many it may mark the end of the road with the BA FPP - however do bear in mind it certainly isn't the only route to the RHS and certainly not the only route into BA.
I wish everyone the best of luck for the future.