Another option would be to try Parkatmyhouse.co.
Go to this link
Abparking.co.uk is the carpark used by RCI - their website is set up for all the cruises. It is approx £109 for a 2-wk cruise.
Mr Aldrich gives the correct web address-the website listed by Mr Ingle deals with Cunard,P&O and Princess.
If you have not booked try for a free parking deal RCI seem do these through some agents,If you have booked look for a deal with hotel stay over night and park the length of your cruise, the easiest is on the dock its a short walk to the ship?