I have just taken a personal plate off my tin top prior to selling the car. It's very easy on DVLA web site.
But what I did not realise is that the new (replacement) number for the car is issued while you are on-line
and you can immediately download a V948 (Number Plate Authorisation Certificate) (valid for 3 days) so you can then go and get a new set of plates made up and on the car the same day. Just a phone call to the insurance company to inform them of the number change and job done - all in a day!
Replacement V5C (new reg cert showing replacement number) and V778 (personal plate retention doc) arrived today (change made on-line last Thur). Well done DVLA.