As a learning PPL - CAP413 does not seem to give any indication as to how and why these non registration compliant names are allocated. Surely registration would be more useflul than flight numbers.
I see there is alot of flexibility at the discression of the controller to avoid confusion as in abbreviating callsigns - but does G-XXXX have to sign on as such following cap413 - or do they just state speedbird xxxx.
Are the callsigns an unwritten rule ? or have I not got that far through CAP 413.