from Calais to London

The Eurostar doesn't stop at Calais, so you would have to go back to Lille to get a train. In about the same amount of time, you could take a bus. Flixbus and others have service from Calais to London.

Flixbus do not pick up and set down at Calais (or Dover). They board/deboard the ferries there, but only pick up in Paris or Lille (with some Lille departures needing a change at Bruges, unlikely as that sounds.)

They take 7 hours from Lille to London due to ferry check in and passage times.

Quite a few trains run from Calais Frethun to Lille Europe in around 30 minutes. A lot more trains run from Calais Ville to Lille Flandres but they take 70 minutes.

Thus, even with Eurostar check in time Eurostar from Lille Europe will always be fastest.

Note that only 3 ferries a day from Calais to Dover take foot passengers (0950, 1500 and 1900 from Calais, 0945, 1340 and 1640 from Dover). You have to get to the port (well out of town) a full 90 minutes before departure. At Dover you are last off the ship, after all the cars as a minibus has to drive on to the car deck to take you off, then a transfer to Dover Priory Station. It takes forever and is just a general hassle nowadays.

your question is how to return to London. How are you getting from London to Calais without a car? What's the plan?