I posted about this in the transport section. Today I had spoke to the GBCC, who issue the ACU carnet’s, and CARS Europe, the UK Carnet issuer.
The results were not conclusive. According to GBCC, carnets are not required for road registered vehicles with a V5, and one vehicle can transport another vehicle - like a motorcaravan towing a car as long as it’s not a commercial operation
Confusion may arise with either more than one vehicle being towed, or taking a few mates bikes in the back of a van. According to GBCC nothing wrong or illegal in this, however, CARS told me anecdotally that a car that was towing a 2 vehicle trailer was asked for a 40% deposit on entry into Spain, and that taking 4-5 bikes may mean customs deem you to be a business and ask for freight and shipping paperwork.
And as it’s a unified approach across Europe

, apparently Spain are being difficult, Netherlands will wave you through and France it depends on the customs officer on the day.
Bit of an arse if you’ve just got off the ferry in Santander and customs ask you for the 40% deposit though, even if they’re not supposed to.
Sent from my iPhone using Tapatalk