I've owned 2 bikes on Q plates. One was my xs1100. The reason for the Q was an indeterminate age. I contacted Yamaha UK and got a dating cert telling me the date of manufacture and the market it was sold (Europe). I presented this at the Glasgow DVLA office and was given a T plate (1979)
The other is my Triumph Speedtwin. Bought on a q plate but with matching numbers. I contacted the TOMCC and got a dating certificate and once again presented this, plus photos of the bike, and the frame and engine number, to DVLA. This time I had to take the bike to my local VOSA to get it inspected. A few days later I was given an age related plate.
Whilst at VOSA, I was chatting to the inspector who asked me "Is the bike rebuilt from parts, or assembled from parts" I asked his intepretation and was told rebuilt implies using the original parts and reassembled meant from a collection of parts not necessarily from the same bike. Needless to say I opted for rebuilt from parts, and as I say, I was issued with the age plate