Leading Q does not indicate the period of issuing aginst to the other letters.
From the English wiki: "The changes in 1983 also brought the letter Q into use – although on a very small and limited scale. It was used on vehicles of indeterminate age, such as those assembled from kits, substantial rebuilds, or imported vehicles where the documentation is insufficient to determine the age. There was a marked increase in the use of Q registrations in the late 1980s and early 1990s, fuelled by car crime.[citation needed] Many stolen vehicles had false identities given to them, and when this was discovered and the original identity could not be determined, a Q registration would be issued to that vehicle. It was seen as an aid to consumer protection." (http://en.wikipedia.org/wiki/Vehicle_registration_plates_of_the_United_Kingdom,_Crown_dependencies_and_overseas_territories#1983_to_2001 )