OK. As no-one's objected to the date, full steam ahead for 11th April.
Departing 06:37 St Pancras HS1, arriving Dover 07:41 (or make your own way to Dover if more convenient). Then get the 09:25 P&O sailing* arriving Calais 11:55 local time. Cycling south from Calais, having lunch somewhere, then heading off west to the coast around Wimereux, then following the coast road all the way round, back to Calais. Depending upon time, either have something else to eat in Calais, or eat on board the boat. It'll be the 20:35 boat arriving back in Dover 21:05 local time, to get the 21:49 HS1 arriving in St Pancras 22:54.
P&O Ferry tickets can be booked
here. Although if you're on Quidco, go via their web site as you can get 6% cashback but that may run out after today. Also the Fully Flexi ticket seems to be the same price as Standard at £20 return, so go for that option, as you can cancel without penalty if you change your mind.
DON'T FORGET TO CHECK YOU'VE GOT A VALID PASSPORT (and make sure you bring it)!!
*
Bearing in mind it only takes 10 minutes to get from the train station to the port in Dover, although P&O say you should check in 45 minutes before departure, it's entirely possible they may let us on the 08:25 sailing.