We travelled with LD lines from Portsmouth to La Havre. The crossing was overnight leaving Portsmouth at 11pm and landing in France at 7.30 am. We basically had a quick drink and settled down on the lounge floor to sleep. There was plenty of room and we were not disturbed. If the thought of sleeping on the floor isn’t to your liking then I would book a cabin rather than the recliner seats as quite a few in the recliner seats ended up sleeping on the floor anyway. No problems with the crossing at all.