Stena Line runs a Rail Sail to Holland offer known as the ‘Dutchflyer’ where you can take the train from London Liverpool Street (or any National Express East Anglia station such as Cambridge, Colchester, Ipswich and Norwich - for the full list see nationalexpresseastanglia.com) to Harwich International port then catch either the morning or overnight Stena Line ferry service to the Hook of Holland. Here's a video on what it's like to take the Rail Sail to Amsterdam:

(Video Copyright Greentraveller Limited)

There are also regular rail services to Harwich from various destinations across the UK via London and major stations in East Anglia (such as the cities above). Harwich International Rail Station is immediately adjacent to the port, so it is a quick transfer from the train platform to the ferry check-in.

Harwich Port facilities: The terminal building has a shop, café, bar, ATM and bureau de change.