If you want your own callsign, then you have to apply to the CAA for one, and also locate an unique three character prefix - unique worldwide, not just the UK. It is this prefix and numeric suffix that then goes on the FPL in box 7, with the REG/ in box 18
I have a suspicion that due to a certain amount of 'vanity' callsigns, this may now be restricted to AOC holders, but for the definitive answer, all I can suggest is the CAA
hth
IFR