For our trip to London with friends and subsequent visit to our cousins in northern England we decided to take the ferry across the North Sea. The ferry worked out pretty well with a two year old. You board the ferry at 8pm in The Netherlands and arrive in England at 6am. You hop a train (which is included with your ticket) and you are in London by 7:30am.
Our quaint cabin.
Although there was a casino, movie theatre, and many restaurants on board our daughter preferred the play area.