This is a problem that affects many councils,but primarily those on motorways or dual carriageways.
Such advertising IS illegal,but here we come to the problem.
Although such advertising contravenes planning laws it can take the council some time to enforce any legislation.
Usually the council has to go to court to get an enforcement order, which takes time,and also costs money.
While all councils in principal say they will prosecute many do not (for the above reasons)
Not only that,but when the advertising offender is notified on the intended prosecution they just drive the hoarding somewhere else,the councils really cannot win,especially given the fact that if the mobile hoarding is moved into the next field a new enforcement order is needed,and so it goes on.
You will realise then that many councils turn a blind eye to such advertising,even though they would deny it.