Tidal streches of water are open access so if you can find a public slip way / area to launch on its free to paddle, Beaulieu river is the only exception to this rule as the estuary is privately owned by the Montagu Family.
Access to inland water ways and lakes must be oked in advance as it up to the land owner whether they let you on or not. For up to date information on where to go see the UK rivers guide book.