No DVLA dateless registration contains the letter 'Z'.(except Special Issue Diplomatic Plates)
Dateless plates cointaining the Letter 'Z' are 'Irish Registrations'.
The number '2' when used in a normal registration looks just like the Letter 'Z' though. Hence some registrations containing a number 2 can immediately be recognised as words, therefore making them higher value combinations!
The current style of DVLA number plates (where the year of issue is indicated by the numbers) does allow the use of the letter Z! (This is only for the 3 random letter side and not for the 2 letter local area tag. This goes for select and auction plates too).