Julie, with your trip/flight itinerary, I'd consider the Harwich to Hoek boat journey. Did it last year and would consider again with caveats and comments.
*avoid weekends as a bus often replaces the train for at least part of the trip London to Harwich.
*go the extra few $'s for a suite rather than a berth cabin. The top-of-line suite includes large cabin & beds for two (not bunks), wall to wall windows, easy chairs/table, stocked and gratis mini-bar & fruit basket. The difference in cost for a couple is minimal (see the ferry website).
*Boat leaves Harwich at 11PM+/- so, evening train from London to Harwich, board the boat about 10PM, arrive Hoek at 8AM+/-, train to A'dam from Schiedam/Rotterdam arriving about 11AM+/- @ A'Dam Centraal. We were at our Den Haag hotel by 10AM+/- last year & A'Dam's about another 50 minutes+/-.
*You've saved a nite's hotel cost, won't be backtracking as you would if Eurostar/Thalys is used via Brussels/Paris, and no airport security/luggage hassles if flying Ldn to Schiphol. Pre-underwear bomber and lightly packed I'd consider flying, not now with all the new rules.
*The train/boat/train connections are slick and immediately adjacent to each other at both ends.
*Decent restaurant onboard for late dinner and breakfast (+ lots of duty free if interested).
*Can buy req'd Holland train tickets onboard the boat.
*It's a fun, different experience and a bit of a break after 3-4 days in London.
Have also done the Kingston-Upon-Hull to Zeebrugge boat which was better because it departed & arrived in daylight. The Harwich/Hoek boat departs in the dark so nothing much to see out all those windows (except a lot of lights onshore and from the oil rigs) until you arrive at Hoek in the AM. No regrets though, enjoyed both trips and a good option depending on itinerary considerations.
Have fun.