also the DVLA have started stating on V5s on some cars Number plate Not Transferable i sold a Rover 800 Vitesse in the summer that had a private plate P9 GR* and the logbook stated number plate nit transferable 
It will depend on a number of things, I believe, including if the car becomes accident damaged/insurance total loss whilst the private plate is on and whether its been out of tax/mot for an extended period. If the latter, I think the plate becomes irrecoverable, but not 100% on that....
depends how long after such accident/mot expiry occured as far as i know
a donor vehicle needs to technically be roadworthy by DVLA rules for someone to have it's plate removed, because it needs to be re-registered to its original (or available age related) number, but if you crash your car and write it off then its not technically road worthy, so they give a time limit of something like a month to get your plate off a written off car so you dont lose out