Hello,
I have booked most of my journeys at
www.zylmann.de (also website in English) or at
http://frachtschiffreisen-pfeiffer.de/Both speak English as well and have done many trips by themselfes. You can choose a trip from their list, please take time to explore, there are many trips to do. If you do not find anything suitable for you, despite looking carefully, you can write them of course and ask. Schedules in the voyage descriptions start very often at Hamburg and Rotterdam, this is because their customers mostly come from Germany. As we all know, ships call at many ports, so it is as worth to ask, whether you can for example start your trip at Felixtowe etc.
Prices vary, depending on the ship, depending on the route, from 80 Euros to around 100 Euros.
I hope, you find something and I bet, you'll enjoy it