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.
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.