I've done this before with my Clio RS, basically when you sell the car you fill the plate retention/transfer form in with the person basically saying that they're handing the plate over to you as detailed below, you will become the grantee-
How long do you want to retain the vehicle registration number?
One year: £105
Two years: £130
Three years: £155
If you are retaining the vehicle registration number, as the
registered keeper do you want to become the Grantee
(the Grantee is the one who has the right to the vehicle
registration number)?
Yes
No
If you
do not
want to become the Grantee, for example you are
giving the vehicle registration number as a gift, give the details
of who should be the Grantee in the box below.
This means
that as the registered keeper, you will give up the right to
the vehicle registration number
You keep form and send off with the fee, they'll then get notification to change plate over and you'll receive the plate certificate. Very simple. Bear in mind the DVLA for some reason will not tell you that you can do this even though the form is specifically set up for it.