It sort of happened gradually in as much as more and more of it was done without question
So in the end they made some decisions
They felt in a modern world exports could benefit and so we had more and more out of sequence issued or some just given a current mark to give the impression they were new aircraft
They thought of stopping it but then decided to allow it with an extra fee for out of sequence.
Out of sequence also allowed retrograde period mark for example the Atlantic Bristol Freighter recieved G-AMLK in the mid 'eighties. This airframe was one of the small number of Bristol 170's built that had never been allocated a UK mark on the line
The mark was a genuine Biristol 170 31 issue but had not been taken up as it was converted on the line into the MK 32 Superfreighter prototype and re registered G-AMWA.
Most of the Silver City Superfreighters were re registered to follow a sequence G-AMWA-WF, NWG-NWN and could be re registered because the mark had not been taken up.
In reality there were a small number of re registrations throughout but there had to be a good reason as mentioned above and there were even out of sequence issued... for example the ATL Accountant was given the marks G-ATEL for Aviation Traders Engineering Ltd. years before it was due. G-BSST for the Concorde and the two Harriers G-VSTO and G-VTOL were also issued before it became the norm to do so. Probaby the first issue or special marks was even before the current register when the Directorate of Civil aviation used the marks G-EDCA on two different aeroplanes, a DH60 Moth and a DH.60X Moth......so the authority was quick to break their own rules.
Just a few note while on the subject
After G-EBTQ the letter Q was no longer used
For those new to the hobby the earlier G- sequence mentioned above was as follows
G-AU** Australia
G-C*** Canada
G-CY** Canadian Miltary
G-E*** UK heavier than Air
G-F*** Lighter than air (Airships)
G-G*** Gliders
G-I*** India
G-KA* Kenya.only one registered, G-KAA a DH.51
G-NZ** New Zealand
G-U*** South Africa
Garry