If its a Porsche specific one and you are in no hurry to sell... Try private ads here on PH, 911UK.com, 911 & PW magazine or via the Porsche Club GB and their Porsche Post magazine?
I sold one through Elite Registrations a couple of years back. They gave me the choice of an outright purchase price, or an SOR basis for a higher return.
i have one at the moment ( PO55RCH ) & don't know where to AD. i had it some time ago on an atlas grey 3.2 Boxster S. It looked quite good. at the end of the day just a fun plate really
I've sold through New Reg, following which I wouldn't touch them with a bargepole, and also Primo Registrations, who are small but give good service and generally lower margins (good news whether buying or selling).
Unfortunately its not Porsche specific, just related to my initials. Seemed a good idea at time but obviously more restricted on resale. I'll try the sites mentioned. Thanks.