The actual offence wording is " Fails to conform to regulations" , which also includes having no plate fitted, ( see link from Jonathan) but is also is referred as "Miss representation" particularly when font, size and spacing is altered and fails to conform. The real world test is whether the plate can be easily read by eye or an ANPR camera and prosecution in such cases is often a matter of the discretion of the Police Officer who stops you. Of note, you only llikely to be stopped for this offence by a Traffic cop as local guys and girls are either too busy or not familiar with the regulations. The offence is dealt with by way of a non endorsabe ( ie no points) fine of £100 if dealt with at the roadside; you would receive paperwork and later a conditional fine of £100 in the post.( conditional in that you must admit the offence and pay, if not you get a court date) A referral is sent to DVLA and if the same driver continues to drive with an illegal number plate, ie is caught twice or more and DVLA receive multiple notifications, DVLA can rescind the number plate and Re issue an age related plate to the car. This is a particular ouch for someone who has spent thousands on a private plate and has altered it by font or spacing for example especially as DVLA can sell the plate again to a new owner and are under no obligation to give a refund to the original owner. Lesson being, don't take the whatsit. If there are aggravating circumstances, for example multiple offences or the plate has been altered by say black tape for the purposes,of avoiding detection by speed cameras or ANPR, then the matter can go to court and the maximum fine increases to £1000 for the plate offence, whilst other offences such as Obstruct Police or Pervert the Course of justice could be considered.