I've been talking to some acquaintances in the car trade and also to my son, who works for a "well known" company that offers to sell you a data check on your car to make sure there isn't finance, etc, outstanding. They try and check for Police markers, but with very limited(5 to 10%) success.
My conclusion, after talking to all parties, is thatwhen buying a used car once you know the registration number you should do exactly as you suggest, and ask the ANPR team at the County/Regional police HQ if there is a marker.
I'm told that some forces are helpful, others are not. In some cases a Freedom of Information Application is required....
As ANPR is used more and more I believe that unless there is some way to check easily, for example, a public website where all car registrations that have a Police market can be checked, buying a used car will become more and more like playing "Russian Roulette". Being stopped by Police who think you may be armed and dangerous isn't my idea of fun.
Best to stick to a Morgan....