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Back to the original post, the OP presumably doesn't know the registration numbers of all the cars he has ever owned.
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No he obviously does not. That is exactly what he wants the DVLA to tell him:
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I would like to know what information the DVLA holds on me, in particular, every car I have owned (at least 20) and when, just because I cannot remember some of the registrations and would like to look them up.
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The information they hold on him is simple: they hold details of his licence (classes of vehicle he is permitted to drive, etc.); his driving record (endorsements, disqualifications); and his current address (so far as he has told them). That is about all they hold on him.
They also hold details on vehicles. They do this by registration mark. They hold details of its current registered keeper and the address of the keeper (so far as he has told them). There is some other stuff as well which need not concern us here.
The problem for the OP is that there is no link between these two sets of data. There is little doubt that somebody who knows their way round the DVLA database (and who has the appropriate level of access, the time and the inclination) might be able to trawl their way through it to find out, perhaps by addresses the OP might provide, of vehicles historically registered at those at those addresses (though the success of that would depend on how long they retain the data). But it is something, I would suggest, they would not be prepared to do.