You can google with the words "ferry London Netherlands"
Ferry companies are Stenaline or P&O Ferries.
I found a good all-in package at http://www.seat61.com/Railpass.htm for 29 GBP, including train from London to Harwich, ferry to Hoek van Holland and from there a train to any Dutch city.
Now from Hoek van Holland to Kapelle it's just over 2 hours with public transport.
For train info check the planner at www.ns.nl (click the English link at the top). The train station names are:
-Hoek van Holland Haven
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