I can only comment on the Ferries UK / Spain
1) I've never called, just booked on line -7.5 m plus 2.5m trailer
2) They have lines painted on the tarmac at check in and quickly check the length. Do NOT understate, they charge extra if you do. Remember a bike rack or tow bar are part of the length.
3) Staying is MoHo is not allowed, the vehicle deck is behind closed doors during the voyage. I had to get medication from the van during one trip. The purser gave me permission to collect it but warned me that in the event of a incident I would be sealed on the vehicle deck even if that deck was being flooded. I was escorted there by a crew member who repeated the warning and said his job was to prioritise the safety of the remaining passengers irrespective of my well being.
4) The ships I have sailed on had dog friendly cabins and kennels. Dogs were not allowed to stay in MoHos.
Maybe worthwhile re reading BF documentation or contacting them.