Never a problem with P&O (Hull-Rotterdam/Zeebrugge) crossing.
DFDS though, pedantic to say the least. Problem was only outbound from Newcastle, with return from Amsterdam check-in staff not batting an eyelid. Landrover with 2 bikes on roof extending 10cm above the height of caravan attached to the back of car. Check-in staff asked for ~£60 extra for bikes as we’d not added them to booking (our mistake) as “you’re too tall for the ferry”. Tight Scottishness won and I took them off and stuffed into caravan for ferry.
Got on board ferry and we – along with every other caravan – are in same section as coaches/lorries with LOADS of headroom…