Lots of good points mentioned above. Regarding the cost to BA to run the recruitment process; yes it might not cost a great deal to empty the pool and start again but in this day and age of cost cutting, you do wonder what's the point of incurring any additional unnecessary costs though?
Having failed a couple times prior to being successful this last time I do know for us candidates it's certainly not easy to pass, but I wonder what their logic is regarding our ability to do the job if after 18 months they don't offer us one. Provided we are still working in our current airline and having already passed BA, what exactly do BA think has happened to our ability in this time? As far as I can see we've only gone and gained even more experience and are bringing even more to the table.
It is of course their train set, and while I'm still positive I do know two things;
1. I'm happy I passed and am at least in this situation instead of having failed again, and
2. Having passed I now know then level you need to be at to get through. Should I need to do it all again, while I won't be overly happy, I'm pretty sure I'd pass.
FWIW I expire in early March 2018.