Keeping prior experience under your hat is not sensible. As a QFI, there is nothing more frustrating than teaching people stuff they already know, which is a waste of everyone's time and resources. If you need to train bad habits out of people, so be it - I had to go through the process myself when I did the QFI course, having drifted into using a couple of 'unusual' techniques on the front line! If you think you are God's gift to aviation and you are not, it will quickly become apparent to all concerned, and you will either get over it or find yourself following a different path.
And the one place you really do need to admit to prior experience is as part of the OASC aptitude testing. If you were to omit to mention your PPL your aptitude scores would give a false picture which may give you an unfair advantage in what is a very competitive selection and where 1 extra point can make all the difference. If that information subsequently comes to light, you can expect your service to be terminated on the basis that you lied to secure employment. There are plenty of young people who have the required honesty and integrity who will happily take your place.
On the other hand, the Service needs to recognise when people have valid experience and adjust accordingly. One of my FJ colleagues had joined with 800+ hours of operational experience (counter-insurgency) in Africa. The rigidity inherent in the FT system at the time meant he was required to start from scratch at BFT which, not surprisingly, he waltzed through. Nobody had the sense to spend e.g. 10 hours on a full assessment and then decide on what training he actually needed.