For the technical ones that like to be PC refer to dvla INF104 page 9 which clearly states the below copied rules directly from VOSA.
They may be 'Illegal' but as the software used to print the plates at work are formatted by Vosa which will not allow for silly plates to be made with incorrect spacing or obscene words etc.
For vehicles with new or replacement number plates fitted from 1 September 2001, registration numbers can no longer be shown over three lines (unless the vehicle was first registered before 1 January 1973). Also, the letters and numbers (characters) on number plates bought since 1 September 2001 will need to meet the following standards. n Characters must be 79mm tall n Characters (except the number 1 or letter I) must be 50mm wide n The character stroke (the thickness of the black print) must be 14mm n The space between characters must be 11mm n The space between the age identifier and the random letters must be 33mm n The margins at the top, bottom and side of the plate must be 11mm n Vertical space between the age identifier and the random numbers must be 19mm