PS to my message above.  Are there any later ferries? That would help.

I just added these words above but can find no way to delete this one!

It’s shown as fully booked but I’m pretty sure it’s wrong. My guess is rather that P&O quietly stopped selling foot passenger tickets. In the last few months it was only a single departure per day while there are dozens of boats full of cars.

Since Dunkerque is also unavailable to foot passengers you’ll probably have to leave from Dieppe... :(

Let me find something.

There is a midday departure from Dieppe for 39€ (DFDS). You could spend the night before in Rouen for example.

Foot passengers are only allowed for the 10.05 / 13.45 / 17.25 departures from Dover and 10.35 / 15.50 / 19.00 from Calais. Capacity is limited because passengers are driven by bus to the ship 90 minutes before these departure times in Dover or back when arriving there. If there’s no bus, there’s no availability. For May 21st the 10.30 and 15.50 departures from Calais do have availability. Book here: 

Reserve Maintenant | Calais à Douvres | P&O Ferries (poferries.com)

Thank you so much for that rapid reply. If Dieppe implies Newhaven, I may prefer taking the Flixbus to London and if possible slip off the bus at Dover, as that is where I want to be.

Also, as I understand from my research, DFDS does not take foot passengers. Could Dieppe be an exception?

See reply above; it’s possible to go by ferry from Calais to Dover as a foot passenger.

Thanks @Hektor. I’m pretty sure it didn’t work an hour ago (weird!).

@BrianL Here’s a little guide: https://www.seat61.com/trains-and-routes/london-to-paris-by-ferry.htm#London_to_Paris_via_Dover-Calais

(Indeed DFDS accepts foot passengers from Dieppe, might be useful for next time.)

Thanks very much both thibcabe and Hektor for all that useful info.

I still find the P&O a real pain to use, including via Hektor’s link Reserve Maintenant… and have provisionally booked a 15:50 departure with Direct Ferries. If I learn how to overcome the difficulty of using P&O’s site I can avoid the booking fee. Is it too much to ask for help on that here?  (I will not be using my Interrail Pass on that day.)

Of course you may ask further questions. But for me it does work very well from that point linked above.Unfortunately it’s not possible to provide a working link in english; it always does go back to Dover to Calais.

Here is a very important piece of information from calais-dover.com: