Parking a boat,trailer or caravan on the road is not in itself illegal. contrary to what some people say it doesn't need a registration plate either, in the same way a bicycle doesn't require one because boats, trailers and caravans are not registered with the DVLA therefore have no registration number to display. It does need to be facing the right way with reflectors to the rear.
So the technical answer is yes it can remain parked there without breaking any laws.
However, if it could be claimed to be causing an obstruction the council could become involved and, if no action was taken by the owner, the could have it towed away.