This is interesting. My family are in and out of dozens of 'uninsured' (and by that I mean, there is no specific policy relating to that vehicle in force) but taxed vehicles. Both of my parents are part of a business policy that covers all vehicles owned by the business for business and something like 'short term' personal use, with 'long term' personal use requiring the vehicles to be specified (not sure on the exact thresholds). The reality is that my dad, for example, may change the car he is driving to and from his site or a customer or an auction intra-daily.
I only hope that when a car is sold to a dealer (which the DVLA are notified of) it will be taken off of this automated mailing list, due to the frequency with which dealers will have insurance policies covering multiple unspecified vehicles.