I towed my car on an A frame through France to Spain (La Manga) last December and back again in February of this year. No problems whatsoever from the police.
Insurance wise Comfort Insurance will insure your motorhome and the car being towed on the A frame. NFU will also insure your car fully comprehensive when it is being towed on and off the A frame.
As a matter of interest who, on Motorhome Fun, has been stopped whilst on the Continent for towing on an A Frame and subsequently fined and told to unhitch.