Or just swap your plate for one from a same make and model you found on eBay. You can trigger ANPR's all you want and even the police vehicle mounted ones won't be cause for much suspicion.
It's not possible to legally purchase a plate for a registration you don't own. About 10 years ago the DVLA started to crack down on places printing plates without changing eligibility ahead of time.
I know we're already talking about the law, and there are always ways around this, but you're probably better off sticking a foreign plate on the car.
I find this type of thing the easiest place to purchase number plates because they don't make you jump through a load of hoops with sending copies of the V5.
I did say legally. The Road Safety Act 2006 requires suppliers of number plates to be registered and, looking at the law, I see no provision for the sale of "things that look like number plates but they're really not, honest guv".
UK statute is painful to read and interpret though. I may have missed something.
You implied that the illegality of purchasing such a plate rested on the customer. I think it is perfectly legal to purchase such a plate, it's just not legal to sell one.