DFDS currently offer a special 5 day, 120 hour, return price for crossings to France. Even though, by my calculation, with the outward sailing is at 8:00am on 20th May, the 120 hour limit is reached by 8:00am on Monday morning 25th May, the fare seems to work for the 6:00pm return.
Below are my suggested dates/times which fall into this special pricing. Thanks to Jason for finding this!
Here's the link to the DFDS offer page https://www.dfds.com/en-gb/passenger-ferries/offers/ferries-to-france/short-breaks/3-5-days