Old Buckie R says:
it comes down to a lot of issues but what I can tell you if you have the same aptitude and skills base as the guy standing next to you , but you have a little more knowledge and experience than the next guy
And I say, "CSD is being too polite!"
Folk here should listen to CSD on this one.
PSA have huge amounts of experience in selecting the right stuff for pilot's course. There is way more to consider than Buck R's brief statement. The PSA in the described circumstances is just as likely to say, "If you have the same aptitude and skills base as the guy standing next to you, but you have a little more knowledge and experience because you went out and bought it, the other guy is the preference because he will not have any preconceptions about what to expect on pilot's course, and he obviously has more talent because he is the same as you even though he didn't pay for extra specialist training."
My advice is that if you are interested in FSP, do not go dipping into your pocket for extra training. Sure, if you are keen to be a pilot regardless of the military, self sponsor your licences etc. This kind of enthusiasm may well work in your favour.
Even so, consider and NB: children walk off the streets with no experience whatsoever and make it through screening and go on to become fighter pilots. There is no hard and fast rule because others not quite so young make it through having enjoyed significant commercial training as well.
There is an established thread elsewhere in the forum to pick up tips. Go there and have a long read. Don't go wasting your money trying to buy something the PSA are likely to see straight through. Not specifically wanting to take away a well defined business opportunity from some excellent folk with outstanding instructional skills and wonderful equipment, before money leaves your pocket remember you have to declare all your previous flying, and buying a pre-screening course can be interpreted in many ways, and not all of those ways are favourable.