I wouldn't ring in on every occasion as you rarely see a Traffic cop out on the motorway network any more, but you might get lucky and have one in the area.
If it looks to you like a genuine family heading off on holiday, it probably wouldn't be worth the effort, but if something more got your suspicions raised that it could be a stolen van, different story.
We have towed overnight from Durham to Cornwall & Devon for many years and regularly saw Traffic patrols. The last few years however, we never once saw anything other than the powerless Highways Agency Patrols.
Last year PC printed a letter explaining that since the HA took over the "policing" of the major roads, that some individual Chief Officers saw an opportunity to bolster visible patrols in areas where crime & disorder figures were highest, sometimes resulting in Traffic cops patrolling housing estates to deter criminal damage and hooliganism or industrial estates for burglars.
I suppose this is good if you are unfortunate enough to live in an estate so blighted or if you have a factory that is continually being targeted, but it doesn't help road safety.
From reading some of the earlier comments, the Police are damned if they do prosecute and damned if they don't.
A decent cop should be able to discern between a genuine oversight or a deliberate disregard, however the offence is easily avoided, even if for a genuine reason, you may not have a spare number plate, it's just a matter of removing the rear plate from the towing vehicle and affixing it to the caravan/trailer for the duration of the journey.
Agricultural tractors etc are exempt from the legislation under certain conditions.
Having no number plate would not normally void your insurance.
Ken.