Some cargo ships are allowed to take up to 12 passengers in what they call 'owners' cabins. Above that number the licensing requirements become more onerous, for example they need to carry a qualified doctor.
There are a couple of companies that specialising in this type of travel, here's one I used :
The Cruise People Ltd The Cruise People Ltd [Europe]
88 York Street, London W1H 1QT
For the best in Small Ships, Expeditions and Freighter Travel
Tel: 020 7723 2450, 0800 526 313. Fax: 020 7723 2486
E.mail: [email protected]