AIUI, the CAA approve the use of the 'special callsigns', such as 'Thanet xx', 'Tayside yy', etc. There was even a club that I flew with where the CFI had approval for one of the aircraft to be referred to as 'AirGoat' (and I saw the letter of CAA authority for that one!!)
There are usually specific operating areas outside which these should not be used and the aircraft reverts t a G-ABCD callsign.