I did mine last year, brought a brand new .25 sub 12ftp, already had a alot for one on my licence, took it to my local shop who applied it to my licence ( £20 fee) then i took it home and turned it up to 28ftp.
I checked first with my firearms officer and he told me i couldnt turn it up before it was put on my licence or it would be illegal, also there is no miminum required ftp limit for a fac, as long as it is the correct calibre and serial number on your license all is good.
If you need anymore info let me know
Atb
Mart