Thing is joe, if you finish work and then set off, you mght aswell do it in one hit to chur. If your setting off at 5 you got traffic all the way down to the m25! Your looking at your ferry being midnight, thEn it's half 2.30 ish when you arrive in France, make that 3.30 with the time difference. So then if you get a hotel you'll be checking in at 4 earliest.
Or... You can stay in England. Get a crossing at 6 ish? So by the time you've set off at 4-5 pm Monday woke up and got on to the ferry port, I'll be way into Belgium, probably past brussels. And in would have set of about half 8 on monday night.