Well done WWW. At last someone has used the magic word - aptitude. Your BA CEP 509er at OATS will have taken some pretty stiff aptitude tests and will have been selected on his/her results. A self financing 509er is largely selected on the ability to pay. That says loads to me. This is of course true to some extent of the self improver. Passing the GFTs is of course a hurdle, but my understanding of the required standard means it is not a very big hurdle.
The difference of course is that the self improver will have more hours, and this seems to be what employers look for. It may be that 500 of the 700 hours is tugging gliders at Much Markle, but experience is king.
Last Novembers Professional Pilots seminar at Heathrow saw several low end employers state that whereas they were looking for 509ers in previous years, now they would rather employ self improvers, particularly ex flying instructors as they proved easier to train.
BA have let OATS use them as the carrot in the referred to advert, but they can afford to and have to be be choosy. I suspect that they will take the self improver 509ers if they like what they see in the OATS training reports.