Hmm. I hadn't spotted that #2. That's just STUPID of them.
Plan B. This is a direct copy and paste from Seat61.com website (No infringement intended Mark) :)
How to buy tickets starting in the Netherlands...
You'll need to use the Netherlands version of Stena's website, because their UK website only sells one-way or return tickets starting in the UK. To buy one-way or return tickets starting in the Netherlands go to www.stenaline.nl/ferry/londen.
If you have a Dutch home address & speak Dutch, leave it in Dutch and use www.stenaline.nl/ferry/londen to buy one-way or return tickets from Elk station in Nederland (any Dutch station) to London or any Greater Anglia Railway station. Easy! Amsterdam to London starts at €52 + cabin cost. Tickets will be posted to you, as long as there are 8 days or more before your travel date. If there are less than 8 days to go, you will need to book a ticket starting in Hoek van Holland, the ticket will be emailed to you and you can buy a separate train ticket to Hoek van Holland Haven at your local NS station.
If you don't have a Dutch home address, go to www.stenaline.nl/ferry/londen and switch it to English at top right. You'll notice the Elk NS station in Nederland (Any Dutch station) option disappears, as Stena Line will not send the necessary Dutch train tickets outside of the Netherlands.
So you're going to buy a Hoek van Holland to London Dutch Flyer ticket, then buy a Dutch train ticket separately...
So first use www.stenaline.nl/ferry/londen to book a Dutch Flyer ticket from Hoek van Holland to London or any Greater Anglia station. This covers the ferry and the British train. Hoek van Holland to London starts at €45 + cabin cost.
Anyone from any country can buy tickets this way. You receive a confirmation email with an e-ticket in .pdf format, which is all you need to check in at Hoek van Holland ferry terminal, where you will be given your UK train tickets for the Harwich to London part of the journey.
Then buy a Dutch train ticket separately, at the station on the day of travel, from Amsterdam or wherever you want to start in the Netherlands to Hoek van Holland Haven. Hoek van Holland Haven is the station you want, not Strand. You can check prices (but not buy online) at www.ns.nl. No reservation is necessary or possible for Dutch domestic trains. Remember that Dutch stations accept cash, but not cards. Amsterdam to Hoek van Holland Haven costs €17.
To buy tickets by phone: UK residents can try and buy tickets starting in the Netherlands through the UK Stena Line telesales office on 08445 762 762, lines open 08:30-20:00 Mondays-Fridays, 09:00-18:00 Saturdays, 09:00-17:00 Sundays, though you may end up having to call their Dutch office. If you're not a UK resident, call Stena Line's Dutch office on 0900-8123 (if calling from the Netherlands) or +31 174 315 811 (if calling from overseas), lines open 08:30-18:00 Monday-Friday, 09:00-17:00 Saturdays, Dutch time.