Andy, there will only have been a record created (and microfilmed at Swansea) for vehicles which were 'active' after computerisation began. In some cases, this will have involved the keeper forwarding the old brown log book and they should have microfilmed it before destruction...However, if you know that the old brown log book was simply lost and it has never been on the computer system then that will be a dead-end.
DVLC most certainly didn't ask local authorities for copy records relating to 70 years worth of registrations , mostly for vehicles that no longer existed.
I don't believe there is any evidence that DVLA ever have or intend to re-sell old registration numbers..there are enough permutations of the modern sequences.
Initially, it was quite straightforward to put older vehicles onto the system, but it was exploited to a criminal degree by the parasites who call themselves Number Plate Dealers. I've seen motorcycles with thirty or so frame numbers stamped on them. At one time, they accepted tax discs, but that was fiddled too and now the burden of proof is extremely high..