I can confirm that this can be used for the on any P&O sailing -(that carries foot pax) in the direction: Calais-Dover. Simply buy the Calais day trip ticket for the day you wish to make the Calais-Dover crossing at any UK ticket office -(after convincing them of its existence..), and discard the unused outward portion. No need to book anything with P&O, just show up at the P&O desk at the Port de Calais with the return portion one hour ahead of your desired sailing to check in and they will give you a boarding card to join the port bus outside the terminal. Just make sure your given the ticket back with your boarding card as you'll obviously need that for the train up to London on the other end! As discussed above, valid on any southeastern service into VIC, CHX, LBG, BFR, CST etc but NOT on the high-speed services up the CTRL into STP.
You also should be able to get a cheap enough Super Sparpreis Europa booked well enough in advance from Nürnberg Hbf to Paris Est -(early morning IC to Stuttgart for TGV / ICE), five-ten minute walk over to la Gare du Nord for a Corail InterCité upto Boulogne-sur-Mer, cross platform change for the TER on the last leg up to Calais Ville. There's usually a good enough through connection this way around 13h31 most days, -(IC 2017 if memory serves but check HAFAS or SNCF app closer to the day as TER Hauts-de-France has been known to change the times around lately). I've done it and made London channel tunnel free in a day this way, off the first ICE/TGV from Frankfurt and München, only €15 on FIP..
Then its a nice twenty minute walk -(or there will be cabs waiting outside la Gare if you have heavy luggage), along Rue Royale -(aka thirsty street!), over the Place d'Armes -(plenty of really good places to stop off for dinner or a drink, before catching a later ferry if you like..), along Rue Lamy and over the bridge, onto Avenue du Commandant Cousteau into the port.
On the other end you can walk into Dover town centre and up to Priory Station or there are usually cabs waiting to meet boats outside the terminal.
Indeed you could just about even make London in time for the lowland and Cornish sleepers this way if you plan it right and don't stop off too long in Calais or Dover -(which is always a challenge for me!).
Hope this helps someone, any questions or recommendations for Calais send me a PM.
Bon voyage!
