Mark...Officially, the total shipping length of the car/caravan needs to be recorded when booking ferries and if you are carrying bikes on the rear of the van this needs to be taken into account. I have used dozens of Ferries and the overall length has never been measured on the dockside so there is little likelyhood that it will be picked up.
Remember also, your number plate and all lights must not be obstructed and if you travel abroad, many countries require a square reflective warning board (diagonal red and white stripes) to be fitted as well.
However, my caravan is an Abbey Spectrum, which is a Swift derivative and the only bike carrier recommended for SOME models only is the Fiamma rack. The fitting of this requires the rear of the caravan to be drilled completely through to the inside of the van to fit the fixing bolts and ties....my dealer did not recommend this. He said the construction of continental vans (German mainly) is different to the UK vans and in his opinion the rear of most UK vans is not really ideal to carry bikes. Saying all that he did say it could be done but I would have to live with the fixing bolts through the rear of the van and these would have to be left on when the van is sold.
You also need to compensate when loading your van to allow for the extra weight of the bicycles and the carrier at the rear.