Hi Meggles,
Depends on the Seller, if he's happy to sell the car with the plates then great, you'll then have the car registered in your name with the personalised number plate on it, i.e you will have the right to display that particular mark on the vehicle.
If the seller says he wants to 'retain' the plate, then depending on how the deal goes through, eventually he will 'retain' the right to that particular mark.
Best give him a call and ask if it's included or not!
I like the US system where plates are done state by state, so you actually do stand a chance of having a particular mark or plate you want. Here, number plate dealing is and can be big business and particular marks are worth lots and LOTS of money!
Back to the van in question, if part of the deal, then when you get the vehicle the paperwork will probably include the documents which show the plate was transferred to it previously. You'll then register the vehicle in your name using that mark. I think then you get the form to 'assign' that mark to your name as well. At that point, you could then sell it on if you wanted to.
if the plate isn't part of the deal, then the seller may sell you the van as is, but a clause will say for you to transfer the plate back to them at a later date. (I've done this with one of my personal plates on a car I sold - got the plate back about 2-3 months later) or they may start the registration transfer process now, wait till a 'normal' mark or plate is assigned to it, and then hand it over to you. You'll then register it with the new 'normal' plates.
Sorry if i sound a bit vague, but thats the general gist ! I'm going through moving some of our personal plates between various cars at the moment, and a good mate of ours is doing the honours by doing it for us at the local DVLA office! - should raise a bit of a smile - the plate I'll be having on my car spells out a swear/slang word !
Cheers! DtM West London & Slough UK!