I travelled with a road bike and huge suitcase and they lierally wanted me to ride the bike onto the ferry. After much protestation and a speaking with the supervisor I convinced them that this was impossible and frankly a ludicrous proposition and was allowed to walk the bike on like a normal passenger, suitcase trailing in one hand and the bike in the other.
When on board staff were helpful and I was able to lock my bike directly to the luggage rack inside the little luggage storage room where you keep your cases. This room was then locked for the duration of the crossing. Nice!
It was an expensive carbon road bike so you can understand my concerns.
This was all from Portsmouth to St Malo and I had the same experience minus all the initial faff on the way back.
Hope this info helps. If in doubt put your foot down 🙂
Fr0sty