I'm pretty sure you can Kev. I have a few on retention just now; they extended the validity period from one year to ten years a while back.
You are correct about not being allowed to place a reg onto a vehicle that would have the effect of making it look younger (with some exceptions regarding ancient vehicles).
I would advise buying your wife something a tad older to make the transfer viable. Maybe a silver 2009 Peugeot Partner van - and as it happens..........