Once a trailer of any sort is attached to a vehicle it becomes a part of that vehicle for the purposes of moving traffic offences and it becomes just the same as driving a car itself around with the wrong plate on the back.
£200 FPN if a traffic officer nabs them.
If a non-traffic/RPU/FRU officer stops them they aren't usually authorised to issue the £200 tickets to they'll submit a TOR for court instead, and in court its up to a £1000 fine, although in my experience the courts typically stiff the villain for between £200 and £500 unless they have form for it, and that's just for lazy people who can't be bothered to spend a tenner on a plate when they borrow a trailer.
Certain other sectors of society do it willfully along with all sorts of other wheezes as a means of trying to avoid properly documenting Multiple vehicles, or to avoid traffic or zoning penalties. That's getting harder to do as ANPR is getting cleverer and knows that a registration that pinged in, say, Norwich at 9am could not possibly be in Greenford two minutes later and will flag it, so more are being caught and that wheeze is getting harder to pull off. Depending on the circus and the intent they could equally be knocked off for fraud if avoiding civil penalties, or perverting the course of justice if avoiding criminal sanction. Fair chance of a suspended sentence or worse for either.