All you have to do is report to the Police that issued your FAC the details of the rifle you want to put on ticket. I believe that modifying your rifle yourself is perfectly OK. In my understanding, it is the reporting that is important.
In my opinion there is little point in using up a slot on your FAC for a rifle running at just over 12fp. Why bother? Make it worth while or don't bother!
You can take rifles 'off ticket' at any time but remember that the Police are not OBLIGED to give you the slot back although I have never known them refuse.