Here are a few links which you may find useful/interesting:
British Canoe Union
Lots of general information relating to all branches of kayaking.
Shoreham Sea`Kayaks
Stockists of the Kaspian range. Demo boats available
Kayaksportuk
General information and more links
Easytide
Admiralty tide predictions for various ports. Very useful!
Sea Paddler
Lots of useful information related to sea kayaking
UK Sea Kayak Guide Book
Dedicated to sea paddling. Active forum, almanac, trip reports.
UK Rivers Guide Book
General kayaking info and an active forum