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If you go for the night ferry (and this isn't a cheap as possible trip) then I'd book a cabin. There might be some spots to sleep but you'll struggle to find somewhere comfy and quiet. Lots of other people will be looking for the same as you so if you're not the first on the ferry you're competing for a limited resource!
With a cabin you will have your own bed and the ability to close turn the lights off, plus privacy and (hopefully) quiet. Possibly even a shower which could be good if the weather is grotty on the trip down from London.
If you're looking to save money then don't get a cabin, but you shouldn't expect to get a good night's sleep.
Need some help with this two options to cross to France:
1 London to Portsmouth on the same day, wild camp somewhere near Portsmouth, get up very early to catch the first ferry to Cherbourg in the morning (9:00).
2 London to Portsmouth on the same day, make some time visiting the town, have dinner and then take the night ferry (11ish I think) to Caen (arrives the next morning).
I reckon the second one is the best choice but I don't know if it would be possible to sleep during the trip. Are there any comfy sofas or beds on the ferry?