From CAP 427:
4 Exempting Qualifications
4.1 The holder of an Air Traffic Controller’s licence who has, within three years
immediately preceding the date of application, held a Unit Licence Endorsement in
respect of the aerodrome control (visual) rating or of the aerodrome control
(instrument) rating is exempt from all requirements for the issue of a FISO licence.
4.2 The holder of a UK Air Traffic Controller’s Licence who has, within three years
immediately preceding the date of application, held a Unit Licence Endorsement in
respect of a rating other than aerodrome control, should contact the ATS Licensing
Section to determine what requirements need to be satisfied for the issue of a FISO
licence.
4.3 The holder of an RAF or RN ATC Certificate of competence endorsed in aerodrome
control during the three years immediately preceding the date of application is
exempt from the written examination in Navigation and Meteorology. A copy of the
relevant military certificate of competence should be submitted with the application
for the issue of a FISO licence.
4.4 The holder of a current UK Flight Crew licence is exempt from the written
examination in Navigation and Meteorology. Details of the licence should be
submitted with the application for the issue of a FISO licence.
As previously stated, the licence holder must still undertake the min 40 hours OJT and validation before "going solo".