There seems to be quite a big selection of Marina's/Dry Stack's in the Southampton area, however they vary in price, some dramatically.
Trafalgar, Shamrock, Saxon, Kemps, Mercury, Southampton Dry, Hythe, Sparkes, Swanwick, Gosport, Port Hamble, Hamble Point, Port Solent, Hamble Yacht Services, Town Quay, Ocean village, Universal and even Lauren Marine.
Lauren Marine has moorings for a 23' boat at about £1100 per year, can't be deep water at that price though and Southampton Dry Stack are doing deals, is that because they are right next to the rubbish heap?
Trafalgar looks good though!